And here's my attempt at the 'lock on' weapon. First of all, I hated this weapon in Tribes 2, mostly because it made carrying flares a must and put a backseat on using the other available grenade types.
My implementation would be this: The 'lock' would still lock on to certain heat signatures (say, a Legionnaire activating his rocket or going into OD), and it would only sense the enemy's 'heat signatures'. To activate the lock, your reticule would need to be on the subject for 2-3 seconds while they're emitting 'heat'.
The weapon would only be available to sentinels due to it's large, cumbersome size.
The projectile itself would be large, slower in speed (average skiing ability would outrun it), and cause a moderate amount of damage. It can also be destroyed by gunfire, be it a well placed rocket, or several chaingun shots, it would be destructible.
It's also easy to trick, if a capper has a projectile coming in on him, a teammate can go into OD and steer the projectile towards them, instead of the capper. Basically, the projectile will follow whatever enemy is generating the most 'heat'.
I like this type of lock on weapon, because it's not overpowered (it has a lot of cons), but it's also useful for defense and offense, as it will decimate llamas (their jet signatures and slow movement would be an easy lock and hit for the weapon), but it's also not too overpowered against cappers. A well done OD cap and subsequent escape would easily be able to outrun the projectile.
Offensively the weapon would primarily be used against enemy base assets.
Maybe let it speed up if it gets behind someone and into their OD stream. That would turn it into an effective anti-OD rocket but not over power it vs anything else.

Cool idea though.