[quote=Everblight]
Is the Bossonid Empire up for selection?
[/quote]

You can play it, but it's very much on the fringe of things and not the focal point of the roleplay. Even though the Bossonid Empire is an important international player, it will see less action than, for example, the Attalans or Osurogans.

For example, in claimant 6's description it's stated a truce and subsequent peace have been signed between the Imperium and Bossonia.

A good compromise (if you're set on playing something Bossonid or Erthanian), would be to create a character in service to one of the claimants as a general or leader of Bossonid mercenaries. Or a Bossonian prince or emissary accompanying someone.