IMO its feasible/legal per RAW.
The description for Gentle Reposes states "You preserve the remains of a dead creature so that they do not decay." with RAW defining "dead" as being when "The character’s hit points are reduced to -10, his Constitution drops to 0, or he is killed outright by a spell or effect." (emphasis mine)
The key word being -OR- because even though liches have recovered their hit points, they still have a zero Constitution so are dead per RAW and thus subject to Gentle Repose.
There is a difference between "Zero Constitution" and "(null) Constitution". A undead creature has no Constitution score at all. It has no value assigned to it, not even 0.
That being said, I would be inclined to employ the Rule of Cool for this one. I don't think it works by strict RAW, but it's a clever use of an otherwise mostly unused spell, and should therefore be allowed.