TCLights v2.4 (Siren/Strobe Program)

[[TC]»Elmo«]

If you choose a cycle mode with bullhorn available, you can have fast/slow/5 in the cycle (which is how it works in the UK and probably other places), 4 using the dedicated bullhorn button and 1,2 or 3 as the normal LFS horn button (aka hold down the TC Lights siren key).
I forget exactly how it works, but you may be able to bind LFS to use either 2 or 3 and let the TC Lights combo key use horn 1. Can't remember if TC Lights uses whatever you've set in lfs or forces horn 1.
You can also set the built-in LFS horn using /horn x

Not quite what you're after, but handles most of it in a roundabout way. I do have plans to introduce several more dedicated buttons in future when I get round to it.