FWIW, I can now answer the questions I raised in a couple of posts above:
#290 - the SX88+ (and thus presumably the SX88) does transmit both DiSEqC 1.1 and USALS commands at the "same time"(consecutively, I presume) if both settings are enabled.
#295 - the combo of the Spaun relay and the RF-remotely controlled 12V supply for it works fine. The relay switches between the steerable TD88 and the fixed dishes with LNBs connected to the DiSEqC 1.1 10-1 Opticum switch down the garden, and thus I can connect to (say 13E) via either route simply by clicking the RF remote control switch button, with no need to go into the SX88+ menus to make any changes - "Simples"

PS: the RF remote is stuck to the underside of the SX88+ remote with d/s tape, and it has a sliding cover over the button to prevent accidental changes from the switched LNBs to the steerable dish.