Maybe some bees could shed some light on how much HCL IPA can absord.
A 5-6 molar solution is commercially available, so the answer must be quite a bit. Assuming 6M is a saturated solution, 1 litre IPA can therefore hold around 220g HCl (i.e. 6 moles). Taking the density of 6M HCl in IPA as ~0.8g/mL, this would give a (220/0.8x1000)x100% = 27.5% w/w solution.
I think..
As for the substance being 'locked in' the alcohol, I don't think that will happen when you have a co-solvent which the IPA is soluble in and the MDMA HCl crystals aren't. This is the theory behind 'crashing out'; the IPA won't be able to hold as much/any MDMA HCl when it's diluted with xylene/toluene as it would when neat.