I proposed something similar a while ago.

I didn't proposed a to-hit rollfor Markerlights, but each Markerlight would give a "token" to a formation within 30cm. Each token could be "used up" to fire a Guided Missile or to improve one to-hit roll. After atokenis used it is removed from the formation. Unused tokens are lost after the turn.
EDIT: Here is my first draft i made some while ago, which i agree is too cmplex

*Markerlights can be fired under the same circumstances as any other shooting attack (eg broken and marching formations can’t use the Markerlight) but before any shooting takes play.
A formation with at least one unit in range and line of sight of a unit with Markerlights is considered to be marked.
Each Markerlight can be used up to bestow ONE of the following abilities to ONE friendly unit (regardless if this unit is from the same formation which uses the Markerlight or another formation)against the targeted enemy formation. Multiple Markerlights can be stacked so putting a marker on the enemy formation for each Markerlight is recommended:
- add Ignore Cover to one shooting attack from a fiendly unit.
- add Disrupt to one shooting attack from a friendly unit.
- fire a Seeker Missile from a fiendly unit.
- improve one to hit roll by +1 for a shooting attack from a friendly unit.
Markerlight markers remain on the enemy formation until used up or if the targeted formation moves out of range of ALL Markerlights (including those of other formations). In the end phase all remainiung Markerlight markers are removed.