I am aware that we don't give gifts -- or, more specifically, transfer ownership of an item -- on Shabbat or Yom Tov. I once asked a question about ransoming the afikoman and received this comment:
... That issue can be circumvented, I think, by having a third party accept the gifts on behalf of the intended recipients before Y"T. This is OK to do without their knowledge because "zachin le-adam shelo befanav" - "we can do unmitigated good for someone without their presence (or knowledge). – Isaac Moses Feb 20 '12
I understand that this is also done when shuls present a bar/bat mitzvah with a gift on Shabbat. And I have seen the "kinyan on another's behalf" approach used recently by somebody who I know to be careful in halacha.
My question is about the logistics: if one wants to effect such a transfer using an agent to accept in advance on another's behalf, what must one do? Specifically:
Are there any restriction on who the agent can be?
Is there any specific declaration text that either the giver or the agent needs to say, or is anything that gets the idea across ("I am giving this to Ploni"/"I accept on Ploni's behalf") ok?
Can this be done for an unnamed person that can be identified later, such as "the child who presents me with the afikoman"?
Can this be done for unnamed people who aren't identified in advance, such as "any guests I have for Shabbat lunch" (maybe you made up something in advance, like bentchers or something to commemorate a simcha)?
DoubleAA linked this page from Hebrew Wikipedia in chat; possibly there are answers to be found within, but I can't read it.