I assumed this was already the case, or so I was told. [RHA] has always been sure to contact it's allies that it looks to involve in the said siege prior to the siege itself. Allies respond, siege dec goes up with them included. Depending on the IFF of the base and it's location the RP varies based on who does the recon of the base. Ex: Sair pob in 49 a while back was scouted out by the SCRA, reported to the RHA, RHA agreed to use military force.
Reutlingen was scouted/ID'd by RHA, contacted SCRA for military support and the siege dec went up with both.
Including allied factions without any linked RP is already a sanctionable offense.