... but not (quite), it seems, on stock Diablo without rotation with Diablo Turbo but without Community SSU :-)

After posting the previous question I took a careful look at the installed software in both App managers, and then I spotted the difference : 810 has CSSU, 800 did not... My mistake !

After adding the CSSU to the 800 and rebooting, ASUI is back to its normal behaviour... sorry for the corner case :-)

I did spot another niggling detail though (probably nothing to do with ASUI) : on the 800, in Cpanel/Navigation, I can uncheck the standard Maemo battery icon, and check the ASUI one in its place. On the 810 I can't because the Maemo icon is greyed out (and checked), so I'd end up with two. Strange.