This code will select multiple tracks in a bank
Code:
oscsend(OSC_to_REAPER, "t/track/1,2,3,4,5,6,7,8/select/toggle", 1); // select all tracks in currently selected bank (reselects tracks after mixer use)
See this code in context in the AKAI MIDIMIX control surface and in Banned's Peavey StudioMix. AFAIK for a given or imagined OSC string pattern it's only experimentation that will determine whether a toggle is implemented or whether multiple parameters are allowed.