Can't say I'm a fan of the idea that the recharge is generating the most heat.
Heat in the case of missiles weapons doesn't make a whole lot of sense no matter how you describe it. Venting exhaust into the interior itself is just insanity (and there wouldn't be that much of it, really), and the reloading action wouldn't be exactly power-intensive.
The concept does have merits, while at the same time, it really doesn't. After all, moving faster generates more heat (by the book at least, this has never been reflected in a Mechwarrior game, IIRC), jump jets generate heat (again, I don't remember this in Mechwarrior games), but at the same time, an Artemis IV doesn't generate heat, a C3 doesn't generate heat (and that's a *five ton computer*), at least not that I remember.
Ballistic weapons are supposed to generate less heat to counteract their ammunition shortages. Yet a Gauss rifle, which strictly speaking is powered with capacitors, should generate as much heat on the reload as a laser weapon.
In other words, Battletech is extremely unclear on the nature of 'heat', but in all likelyhood it's probably a combination of all actions involved - targeting, firing, reloading. In laser weapons, I'd daresay most of the heat is generated from the firing action itself, however.