what is the wattage rating of a 2 watt resistor in oil?
You're asking if placing a normally air cooled device in mineral oil can create a three fold increase in power capacity.  I think that it's close, but yes.  You should easily achieve going from 110W to 300W.  Although bear in mind, that you would not want to run your 22 resistors at 5W in air.  They would get very hot.  The following is for a typical power resistor.  Look at the SBCH4/5 curve.  That's 250+ deg. C for 5W!  That's not good for your PVC case or your hand.



Mineral oil has 5 times the thermal conductivity of air and 1.7 times the specific heat capacity.  The enhanced conductivity will create a much bigger effective cooling area as the heat flows from the resistor cases to the outside of the PVC tube, which will then act as a much bigger radiator.  This is an example of what three types of coolant can do to a heat sink:-



That's a 8 fold improvement.  Please realise that the above heat flow software (Symscape) isn't Solidworks Flow Simulation, but it feels kinda right.  Also realise that the duty cycle will affect the overal temperature rise in the load.  So shorter bursts can use more power.  It's difficult to say what the temperature rise would be with a 100% 300W duty cycle run continuously.  Some dummy loads have metal cases with fins, so they're effectively oil filled radiators.

If running close to the edge is important and you've access to the internals, can I suggest one of these cheapo thermometers plunged into the oil?



Mineral oil boils at 300+ deg.C.  That's not good for anything.