Update. OK, I've looked into this a little further, and I think (maybe

) I have a better understanding of the issue now. The map that we're having the trouble with uses NemesisX SLV rockets (pretty unusual for BunnyTrack!).
And it seems that the setting in BTPlusPlus.ini called
bAutoLoadInsta=
...is the issue, it's set to True on our server. When I set it to False for offline testing, the NemesisX rockets appear. I had a peek at BTPlusPlus.uc and I think this is all down to:
function PreBeginPlay()
{
...
if(bAutoLoadInsta)
{
if ((!bDisableInTournament || !class<DeathMatchPlus (Level.Game.Class).Default.bTournament) && (!bDisableInNonBTMaps || (Left(string(Level), 3) ~="BT-" || Left(string(Level), 7) ~="CTF-BT-")))
{
RemoveActorsForInsta();
Insta = Level.spawn(class'InstaGibBT');
Insta.DefaultWeapon = class'SuperShockRifleBT';
Level.Game.BaseMutator.AddMutator(Insta);
}
}
...because that seems to tbe the main place the bAutoLoadInsta variable is referenced. So... I'm reluctant to set bAutoLoadInsta=False because on all other maps this works fine (I'm not entirely sure what the implications of doing that are anyway). But I'm not sure this is something I can get around with medToggle afterall.
Apparently you can flip the setting of bAutoLoadInsta by typing:
type mutate btpp bAutoLoadInsta False
...in the console in-game. I don't know if there's some way I can do this automatically.