Thunder LRMs fire like artillery in that they target terrain, not enemies. They deliver a minefield equal to the LRM launcher that fired them (for example Thunder rounds fired from an LRM-5 would generate 5-point minefield, an LRM-20, a 20 point field) to the targeted terrain instead of doing damage to it.
The details about how big the field would be, would be discussed (6-20 m for start, depending for the launcher).
The purpose would be to make NASCARs think twice about circling around, due to possible area control with minefields.
Of course, both possibilites as equipment or consumable may be explored in this matter (but I think that with current PGI capabilities atm, the consumable option would be the best).