I was wondering how sensitive these systems are. We are planning a modification which will involve either moving these antennas or leaving them in place under a radome (the radome is designed for x-band frequencies)
Theoretically it can be done; a radome is an antenna enclosure for a radar. It is designed to have sufficient permittivity at the frequencies of interest. For TCAS, you need to look for 1030 MHz and 1090 MHz (IEEE L-Band). Marker beacons work at 75 MHz (VHF)
It is very important to make sure the TCAS antenna has good line-of-sight to all directions. If not, TCAS will be blind in some directions, with potentially dire consequences. If it is the TCAS directional receiver antenna plan to cover, you have to be very careful; near field interference can cause the directionality of the antenna to be affected, causing misleading position indications on the TCAS display.
So whilst in theory the you can put the antennas in a radome, in practice you have to very be careful and you will require many measurements to validate that the setup actually does not degrade the performance of TCAS.