NPCs don't actually stand back up, they just fade away. Anyways, I just recently found out how to do this thanks to my friend deadpoolxyz (aka The Dream Is Dead) and his list of ped flags. There is a flag that he said stops peds from being busted, but it also seems it just completely stops the ped from being registered as dead. In the player's case, this means the game will never realize the player is dead since he will never really be set as dead (but he will still be on the ground playing dead animations). The flag is 58, and can be set like this:
PedSetFlag(gPlayer,58,true)
If you give me some time I'll work on making a mod that does it for you and lets you get back up. I have two ideas for how you can get back up, tell me what you prefer. Either it can be timed, or it can be like boxing where you have to tap a button a couple times to get back up. Also any other side things I should do with the mod, or is this all you want? If you do have anything else you want me to add though please keep it simple, otherwise it may take a while and I may not have time to do it.