This is one of the big issues I have with my 6600 is the lack of the I/O loop which my wonderful 6700 has. What I was thinking of building up was a DPDT coax relay box that will bypass the filter during TX. You'll want to use one of the AMP TX ports with some TX delay added in to switch the relay.
This seems to be an obvious do not discuss topic. The few times I can find it in search, the only answer from flex comes from the crickets. With other brands that provide schematics, one can usually find an internal coax going off to the receiver. Even without schematics, this is pretty easy to find. For me this is a must have, not a nice to have.