Signal for 25sec. clock when kept on field?

[Scuba_ref]

In WA the BJ for crew of 5 and the R for crew of 4 (only the highest varsity level gets a crew of 5 so we do a lot of 4-man varsity games) keeps the 25 second clock and raises one arm above head when the count gets to 20.

Only 2 of the 14 fields we use have a play clock on the field.

[jTheUmp]

MN does the Back Judge basketball style count as well.

At one point, I think there was a suggestion that the R give a verbal count of the last 5 seconds at the same time... but that was quickly rejected because if the R doesn't see the B start the count, he could be counting "5...4...3... (B whistle... flag)", leading to the inevitable "But the referee was a 3 when the flag came!" fiasco. Plus, the R shouldn't be looking at B when the snap is imminent anyway, R should have a lot of other things to worry about.

Seems weird that IA wouldn't want a visible count, but I'm sure the Powers That Be(TM) in IA have their reasons.