The Customize Script (multisitter box contents) allows you to easily add/change help messages, set a prim (or floating text) invisible on sit...
Here you can find examples of "advanced" or very specific customization, and you can use parts of them to quickly create new scripts, too...
You can remote command Single Avatar MultiSit from a script, using:
llMessageLinked(LINK_THIS, 0, "stRC", "");
Using 0 (zero) and LINK_THIS you trigger the 1st pose of the same prim (on a sync prim, even with nobody on it, this triggers people on other sync prims).
With Single Avatar MultiSit, /h on notecard will NOT show menu on touch, and /k will always play the 1st pose on sit, and never show menu to "unknown avatars" the 1st time they sit.
You can make the pose menu appear (example for Single Avatar MultiSit on same prim: it will work even if you turned off the "touch menu" using /h):
if(llAvatarOnSitTarget()!=NULL_KEY) llMessageLinked(LINK_THIS, -1, "stt2", llAvatarOnSitTarget());
A simple script to use remote command (limited to 12 buttons and working from the same prim of Single Avatar Multisit)
Color changer script when not sitting (how to make it work only for non-sitting avatars, so dialog doesn't "mix" with Perfect Sitter)
Menu button with sit options (see how to add a submenu, allowing to either adjust or change pose, to your script's dialog)
Do things when a pose or animation is used (sounds and expression animations in this case)
Give Object when a pose is used (condensed version of the previous script to give objects)
Make some prims (in)visible when specified poses are used (2 scripts) (make prims appear/disappear depending on pose)
Do things when someone or nobody is sitting on object (on any prim: for a special or multi-avatar setup where using Customize Script(s) is not enough)
Reset zPUP to a frame when nobody is sitting on object (condensed version of the previous script for zpup)
Trigger another prim (or prims) on sit/unsit (2 scripts) (again, for a special or multi-avatar setup where using Customize Script(s) is not enough)
Forced sync of the ones using a single pose when someone chooses a couple/group pose (add to Single Avatar Multisit prims with a mixed setup)
Forced sync as above, with additional (customizable) "sync" on return from couple to single (add to Single Avatar Multisit prims with a mixed setup)
Customized Messages (a very condensed version of the Customize Script, to use your custom messages on sit/unsit)
If you need to use Multi Multisit for the passengers of a "special" vehicle, remember to set up the driver seat (usually root) with Single Avatar MultiSit. Set up the passengers on another prim, and add the "root touch" script on root. If you can, better set up "fake" sit targets to ease passengers' sitting, and disable the Animation command(s) on the vehicle driver script (if it has only one driver animation...).
If you use Perfect Sitter (e.g. on a vehicle) and your script unsits quickly (e.g. intruders, non-owners...) better use a pause (e.g. llSleep(3.5); ) before unsitting (or they may keep animating after unsit; anyway they generally just need to walk to stop the animation).
The following line filters out all Perfect Sitter's link messages; note that your messages must not begin with "st":
if (id=="eeaaee"||id=="beeaaee"||message=="Nu"||message=="sitmessage"||message=="unsitmessage"||llGetSubString(message,0,1)=="st") return;
If you are a programmer and make lsl scripts, you may find useful this (free, opensource) bundle I developed. It is for TextMate, a commercial (trialware) "text editor" for Mac; I saw there is also a beta windows port, but the bundle will need some "tweaks" to be used with it.