The tau cannon seems to have a splash in HL1 simply because (when it’s not at a glancing angle and bouncing off) it leaves a red-hot glowing section of wall which slowly cools back down, from which some sparks “drip” off just after impact. It also penetrates walls of medium thickness in doing so so can be used to shoot through walls, something a liquid-firing weapon would not do.
IIRC, there is also some splash-damage as it passes through the wall, but I might be wrong on that one.
On an unrelated note, in HL1 at some point after getting the rocket launcher, there is a tank at the end of a short road with a cliff on one side and a wall on the other (with a hole in it leading to a small ammo cache guarded by a few soldiers).
Said tank (a Bradley I think) fires rockets, specifically the same kind as the RL, and if you have your laser-guide on, they track to it just like your own ones.
My suggestion is to include that fact to the point that, like in the original, you can guide the rockets to the wall or road for safe passage, and if careful enough, guide them to the road just in front of the tank, such that after several shots, the tank ends up killing itself from the splash damage…at no cost to your own health or ammo (c: