However it'd work, however it'd be implemented, I like the idea. Firebug's definitely lacking in single target DPS. Though I imagine the progression moreso like Sharpshooter's "M14 = crossbow", instead of Commando's
bullpup < AK-47 < SCAR-H.
As in, the thermal lance'd be a side-grade to the flamethrower.
If that turns out to be the case, could it and the flamethrower be carried all at once? Probably not, since Sharpshooters can't carry both the crossbow and M14. But on that note, I'd see myself switching from the flamethrower to the thermal lance at wave seven (when fleshpounds start appearing in-general) and feel justified.
I'll be damned, I like this idea.