Combine 2 circuits to get the best of both? Battery management system
Q1. Combining circuits (charger + indicator)

You can add a LM3915  Dot/Bar Display Driver
 as a competely separate circuit connected to the battery (in parallele with the charging circuit).

I'd use the example circuit in the datasheet. You can derive the VLED supply in the same way as in the second circuit or use a resistor dropper as the data sheet suggests.

Q2. Supercaps to boost start-up current


  I can't get enough current from the batteries I'm using to supply the load (a 24V DC motor), it needs around 2A and the batteries are only supplying around 1A ...  It doesn't need to run for more than say 15 seconds, ever.


I don't see why supercaps should be necessary. Normal 12V lead-acid batteries of the sort used in automotive applications should have no trouble supplying 2A for a short period. Many automotive batteries can supply hundreds of amps for 30 seconds at 0°F.

Automotive Lead-Acid batteries have a current rating sometimes referred to as CCA (cold cranking Amps). This is in Amps (and is mostly unrelated to the rated capacity in Amp-hours).  So long as you don't exceed this there should not be a problem.

For example the datasheet for a Yuasa 1.2Ah 12V battery gives 

Maximum discharge (A) 12 
Short Circuit current (A) 36 


The discharge chart suggests it can supply a 2.4A current (2C = 2 x 1.2 = 2.4A) for 6 minutes (down to 11V).