Bengo,
They wouldn't necessarily need mlitary serials. HHA have done both (though now focus on mil registration) and FRA/Cobham have kept the Falcon 20s on the civil register for low-level anti ship and EW emulation. It depends on your AOC, Ops Manual and other dispensations you have from the ANO and other regulations.
In the US, the 'Restricted-Utility' would likely be the preferred path. Their current aircraft are 'N' reg.....