Suppose, for whatever reason, a goalkeeper is substituted off during kicks from the penalty mark.
Can they later be returned to the field by substituting back on?
No, the goalkeeper cannot be substituted back on.
A goalkeeper may only be replaced by a substitute (or an excluded player if one team had more players than the other at any stage of the kicks) if they are unable to continue. This is taken to mean injury, illness or some other emergency.
Once the goalkeeper has been replaced in this way, they take no further part in the match, regardless of whether the match is using regular or return substitutions.
This is outlined in Law 10 - Determining the Outcome of a Match, Section 3 - Kicks From the Penalty Mark (emphasis added):
A goalkeeper who is unable to continue before or during the kicks may be replaced by a player excluded to equalise the number of players or, if their team has not used its maximum permitted number of substitutes, a named substitute, but the replaced goalkeeper takes no further part and may not take a kick
If the league you are participating in allows return substitutions, as covered in Law 3.2 of the IFAB Laws of the Game, then the goalkeeper may return.
The use of return substitutions is only permitted in youth, veterans, disability and grassroots football, subject to the agreement of the national football association, confederation or FIFA.
Otherwise, the goalkeeper may not return as addressed in Law 3.3
To replace a player with a substitute, the following must be observed:
• the player being replaced [...] takes no further part in the match, except where return substitutions are permitted