I'm sure the BAe146 has not got the legs/endurance for a replacement platform and maybe too small.
I wonder if a variant of the new P-8 Poseidon would do the job, basically all you need is a C-40 fitted out including IFR probe.
http://www.naval-technology.com/projects/mma/
Germany has looked at the Global Hawk (EuroHawk) to replace their ELINT Atlantic's. One was flown from Edwards to Nordholz and tested over the North Sea extensively.
I wonder what the ruskies would think about intercepting an unknown aircraft and finding out it had no pilot!!