So, I've had this happen a couple times now. I'm keeping a player at 99%, training up a special ability or a position in preparation for the following season's Negotiations market. However, one day the "skill point gift" from friends gets applied to this player, and this "gift" causes him to go from 99% to 100%, rendering him basically useless.
Thus, my suggestion is as follows: if a player is training for a special ability or position, the "skill point gift" counts as +1 to that ability or position.
Feedback is welcome.