It isn't going to help unless the other jet has it too. That rules out the RAF front line fast jets. Not sure if it needs line of sight too? If it does, 2 low aircraft on other sides of a hill may not get any info on each other.
Using the normal modes of your transponder is different and is advised. Most platforms in NATO capable of firing air to air stores have an interrogator on board. This asks transponders if they are squawking, and some of them also ask what they are squawking. If ATC know you are about, a crew may interrogate an area and expect to get a reply back before the radar has enough information to build a useable track. Something about your kit transmitting a location rather than a radar pinging out in all directions hoping to hit something.
At low level, if you are using your eyes to avoid the ground, so is everyone else. TCAS would be unuseable unless you were transiting above safety altitude. ATC is at best info only, sometimes nothing at all, see and avoid and be well lit when you are below 500' on approach.