Interesting. What about providing an "againframe" environment (on par
with "appendix", "note", etc.)?

It would use "BEAMER_act" property to set the overlay specification, and
a "BEAMER_ref" property using link syntax to refer to a particular
section. Here's an example:

That would be a nice idea. This againframe is quite used.

And your proposal is much cleaner wrt to the org syntax than my current hack.

Fabrice