I figure I'd have a handy supply of wort once I've brewed, and if 5 hours doesn't get it to high kräusen (that's usually the timing for my starters, though) I could wait a bit longer. 12°-13°P instead of 10°P would hardly matter, and no "foreign" wort. It's just a plan I've had in mind as an alternative to a brand new starter, I think in principle we're on the same page. I will probably try it whenever I let my yeast sit a while. But darn it, I just keep brewing all the time!
EDIT I have 1st gen in fermenter now but not because of a hiatus. The yeast in the jar (only a few generations in) had packed down, and a day later started releasing CO2 and rising up like a sponge. Having never witnessed this behavior in my old friend 34/70, I didnt take my chances that something might be abnormal. (The beer I harvested it from seems fine, abundance of caution.) I almost used the method above with a smackpack, but realized I had just enough time to make a regular starter (O2, not stir) and settle it out before brewday. Old habit, good yeast, in kräusen in ~14 hours at 52°F. Vital enough.