I don't think it is completely about the language. JVM taking more memory isn't the problem with in-memory databases. It is having more data than can fit in memory. Even an in-memory SQL Db would eventually run out of memory, no matter the language it was coded in.
A lot of DB design is around this subject. And so it seems like if you found a way to allow a graph DB data to expand beyond in-memory by allowing it to reside on disk is a really big deal. But, since it would be a big deal, I'd think the big guys with a lot of resources would offer it as an option, or feature. It would be worth investing in.