@14Bore, thanks for responding. It's helpful to know how it was done in EIII. I have EV which does not include Grande-Bande skirmishing. Anyway, your response caused me to go back and read carefully…and I think I have my answer.
The turn sequence for tactical combat for R&E is:
a) Army Initiative
b) Winner Impulse
c) Loser Impulse
d) Bombardment & Skirmishing
e) Bonus Impulses
f) Fatigue checks
So Grand-Bande skirmishing is after the regular tactical impulses. One could be confused since the chapter describing the process is prior to those for tactical impulses and bombardment.
The introduction to the chapter on Grande-Bande skirmishing has this:
"If you chose to bombard first, you must declare in your orders how long that will last before launching your skirmishers."
So, it's just once and it will be at the end of any bombardment. That would make sense with the later section describing the withdrawal of the participating skirmishers either to rejoin their parent units or to the ME holding area.
Note: I've used this in the current play-by-email game I've organized and liked it.