How would a tricycle-geared aircraft be recovered from a tip-up?
While I do not have a specific source to cite, which is always important to reference in regard to any aircraft work, I'm basing this answer on my training as an FAA certified aircraft mechanic.  (There are many more experienced mechanics than myself out there, but alas, none who have done this particular process have arrived yet.)

In general, if you have to work on something that does not already have specified methods from the aircraft manufacturer, the go-to reference is:


  AC43.13 1b/2b - Acceptable Methods, Techniques, &amp; Practices of Aircraft Inspection and Repair


But I don't know if that volume covers this mishap.

I can tell you with some certainty that several inspections will be called for on that aircraft.  The skin and structure of the empennage, of course, but also the main gear will almost certainly require a "hard landing check", since the airframe was adversely loaded so much that the nose lifted.

There's every chance that the event will call for a "tail strike" check, which can be extensive.

If the aircraft were tipped forward in an uncontrolled fashion, the nose gear may come down quite hard, but the fact that the forces involved would be unknown (compared, say, to a landing touchdown at a known sink rate and aircraft weight,) would also necessitate a hard-landing inspection of the nose gear.  

In a hard-landing event, the fuselage structure may also be compromised, and relevant areas are also checked, as prescribed by the aircraft's particular inspection and maintenance documents.

When an airframe ever does something in an uncontrolled fashion, many inspections will be needed to conservatively account for every possible imagined consequence, and that can cost a great deal of time and money.

All this is preamble to the kind of approach a good mechanic will take to a problem that offers little previous experience.  Every A&amp;P can change a tire, balance a control surface, and check for corrosion, but getting an aircraft back on all wheels safely is not often addressed.

We know a few things: 
• The nose gear can support the unloaded nose of the plane (and we know from records what weight range that is). 
• Dramatic movements are frowned upon. 
• The aircraft was fine before that last load was introduced. 
• Personnel safety is a primary concern. (No dangerous heroics!)

Things we don't know:  
• How hard you can pull down on the nose gear before damaging something (they were not meant for such loads!) 
• Whether the main gear are at or over their load limit already.  (If people were watching the aircraft's weight and balance figures, we wouldn't be in this mess.) 
• Whether the fuselage/empennage has been structurally compromised. 
• How fast/hard the nose might come down with the weight distribution corrected - too fast?

From these things, we can determine what NOT to do: 
• Pile a bunch of baggage handlers in the nose until it tips back down.  Yay, concussions! 
• Adding weight (ballast) in general. (Will the airframe be damaged further?) 
• Haul downward on the nose with a winch. (Don't tear the nose gear off!) 
• Jack up the tail from the rear. (We don't know if it will hold.  Probably, but we don't take chances.)

Variables that could be leveraged: 
• Can the cargo be winched back towards the front without endangering anyone? 
• Does this model ("type") have fore and aft fuel tanks in the main fuselage, and if so, can fuel be moved between them (or pumped out) for weight shift, in safety?

We're left with two primary goals: 
• Decrease the weight moment behind the mains in a controlled fashion, hopefully without adding any weight to the airframe. 
• Do not let the nose fall uncontrolled.

My approach would resemble the following, depending on equipment available (or that could be found in the local region):

Provide safe support under the nose of the aircraft that can support the normal full weight of the nose of the aircraft.  We want to ensure the nose cannot come down without our say-so.  A sling is ideal, and the airframe manufacturer will have specifications on how to lift and support the airframe.

In a comment, @TomMcW provided a nice linked visual in this YouTube video: 

                
            

As seen in the video, the best option would be an overhead crane, with a sling approved for lifting that airframe wrapped under the forward fuselage.  

Here's a more extreme example of using a sling to lift an aircraft:


Once the aircraft is in a stable support with minimized risk of unexpected movement, the next phase is to UNweight the rear, in the safest way we can.  That might be to pump out fuel from an aft tank, or winch a cargo container slowly forward, or (the least favorite) manually unload items, using as few personnel as possible.

As weight is removed from the rear, the airframe will weigh down the sling.  You can lower the sling in a slow and controlled way, with the main gear "chocked".

Once the nose is down, sorting out the rest of the cargo and fuel will be trivial, as flight crew can enter and operate internal systems.

But one thing I know for sure: that bird ain't flying today!