Yes Grevilleas are very promiscuous and frequently interbreed. Grevillea rosmarinifolia hybridises with the local Grevillea juniperina the offspring is sold in nurseries as Grevillea Canberra Gem
I'm afraid they are not garden escapes. There are a number of Australian plants species planted along the strip between Federal Highway and Valour park / Mount Majura nature reserve which include Grevillea rosmarinifolia and non-local Acacia species.