i think there should be keep inventory on when you purchase teir 5, i dont usually like it but i use power armor and it would suck to die in lava and lose everything. aka (tekkit legends)
This isn't possible. With the whole Mojang issue that arose recently, this falls under an "Unfair Advantage." Sorry.
However... Since /gamerule keepInventory is a base minecraft command, it could be added to our new shop as a serverwide perk, and best of all, you won't need anything fancy, just a command to toggle it on and off. So, maye, possibly, but it wouldn't be bound to a rank.