Revolution Performance are the only big bore kit providers as far as I know. Using one of their kits will get you another 50cc, and that is for both the 500cc and the 750cc. The limiting factor is the thickness of the crankcase 'mouths' where the barrels attach to them. There just isn't the required thickness to safely increase the bore by more than 2mm.
As the stroke of the two engines is identical at 2.60" - 66mm I think it is safe to assume there must be differences in the upper crankcase castings of the two models, otherwise you would be able to drop the 750cc top end onto a 500cc bottom end. However even given that fact, I find it unlikely that you couldn't swap engines as a whole and put the 750 mill into the 500 cycle, purely because of the increased production costs for HD in having to produce two separate frames. Mind, that's what I think, that's not a given.
Would the ECM work? Not with the standard 500 fuel map. The FP3 could certainly flash a 750 a/f map to it.
Then there are the 'not knowns' to consider. Off the top of my head;
Are the exhaust ports the same size or will you need a 750 exhaust?
Are there other implications of swapping two different ECM's should you need to?
Are the final drive pulley ratios the same for 500 and 750?
Are the swing arm length's the same? - drive belt?
Can the 500 fuel pump cope with the 750's demands?
All nitty gritty things that you will need to sort out and made more difficult by Harley's reluctance to make technical specifications available or easily accessible.
No doubt it can be done, with a bit of hassle. You'd have to make sure you have a good sense of humour to take it on.
I notice what you say about trading in for a 750, but the fact remains it is the simplest and most cost effective option if more power is your goal.
Your 500 must have some value to offset against buying the 750.
And when time comes to sell your 500/750 conversion, you'd expect people to be a little bit wary of buying it, which translate as rock bottom resale. Why take a risk on an unknown when the potential buyer can go to HD and buy a 'proper' 750?