So to make it clear:
The Spirit of the Rhino should work with Boomship.
The reason to assume it works is because the already available interactions with the Cards
Charge + Warsong Commander and Rocket Boots + Woodcutter's Axe.
So even if a Minion doesn't have Rush if you give them Rush on the same turn you summon it the Spirit should work.
With Charge and Rocket Boots on a Festerroot Hulk it gains +1 Attack from Warsong and +2/1 from Woodcutter