I have asked my Sakya lama via email if it is know in the Tibetan Buddhist tradition to temporarily ordain upon the death of a family member in order to accumulate merit for them. He said that it is done in the Tibetan tradition, but most commonly people take the 8 lay precepts (sojong), what I have been calling the Eight Mahayana Precepts in this forum, instead of full ordination, for 8, 16 or if possible 49 days and dedicate that merit to their deceased family member.
I am not sure about inTibetan Buddhism as a whole, and it is interesting that your Sakya Lama says it is possible. In the Gelug tradition I have heard one of my Lamas say that it is definitely not 'our' tradition to take monastic ordination temporarily (shramanera/shramanerika or Getsul/Getsulma) as is done in the Theravada tradition. I never thought about whether that was just a Gelug position. On another occasion HHDL ruminated aloud during a teaching about whether it might be useful for Tibetans to adopt this tradition. He didn't come to any conclusion about it, just threw it out there. However the implication was clearly that up til now it is definitely not
the tradition. Again, I didn't even stop to think that this might differ across the various Tibetan schools.
A personal observation of this Theravada tradition: Of several Thai friends I knew who have done this when they were young, I have to say that only a couple actually really got some real, deep core understanding from the experience. Most saw it as an interesting experience which somewhat strengthened their respect for the Three Jewels (not bad at all!) but it was more of a social obligation than sincere choice to deepen their understanding, hence it was an exercise in merit making and social acceptance - learning etiquette.
Coming back to the Tibetan tradition, in accord with Vinaya in general it is of course possible for one who has given back the vows of Gelong or Getsul without having broken them to retake them later. I don't know if this could be constituted as temporary, but again one of my Lamas said that it should not be seen as such. It is more a provision for those who had kept their vows intact, given them back due to circumstance, and wished to retake them later. He also said that for those who were older (middle age) who took the ordination it was generally consider negative to then give up the vows - but this last remark was more of a comment on social mores than actual interpretation of vinaya.