Actually you can fly the higher speed (300kts) when you're cleared on the RNAV and you just gotta comply with all the relevant speed constraints. That was the understanding that I recall based on my "previous life" experience. The 250kts below 10000' when within 40nm from the airport is solely when you're on a radar heading. And as mentioned by CaptainProp, you do occasionally get a chance to be cleared DTO certain points on RNAV arrival and resume star thereafter.