UEFA Champions League regulationsUEFA Champions League regulations include a section on Trophy and medals, which has three subsections. The only part that directly speaks to medals is 11.03, saying that
[t]he winning team is presented with 40 gold medals and the runner-up with 40 silver medals. Additional medals may not be produced.
There is no restriction on who these medals can be given to or where they might be placed (unlike the replica trophies).
None of the regulations dealing with registration of players under VII Player Registration) then prevent a player who joined a club but was not registered or fielded in a match from being given one of the medals.
In the absence of any further restriction or requirement from UEFA, it appears the decision of awarding medals to individuals is left to the winning or runner-up club.