The wiki article, "Frequent trading policy" contains a subtopic, "Setting up a one-shot automatic transfer" which works essentially as described (the VG website has changed a bit since the wiki was written, but the correct places are easily findable.)
One thing I did was use the percentage method to exchange out 99% (from TSMI) into a muni fund (will happen on Dec 24) and will exchange back in on Dec 26 the correct percentage to leave the muni fund about where it started and get most all of my original amount into the TSMI fund before year-end.
The VG website didn't complain at all except that I tried to set it up for today - it wants a future starting date. There was never a mention of having to wait to get back into the TSMI fund.
So I'll be out of the market for a day or two rather than the usual 60 days waiting.
My thanks to the Bogleheads for all you have taught me!