Roger,
I'm thinking of doing the same thing. On the Tomcat I bought, the cushions in the berth are sewn together down the center line of the boat which makes it even more difficult to access those areas. Where yours done that way? It doesn't look like it but perhaps Janet used a seam ripper to separate them. Also, I think it would be much better to have the seam between the cushions running from port to starboard rather than bow to stern. Since one will typically sleep in that orientation, it makes sense to have the seam run that direction rather than across ones back. That would allow you to lift the aft most cushion onto the forward cushion and would make access to the hatches easier. Finally, since the Tomcat berth is 4" longer than a queen size bed in both dimensions, it might make more sense to have cushions that where exactly a queen size bed in dimensions and then fill the other 4" in with some storage system (a shelf, a "headboard", whatever). That would allow one to use standard size sheets (fitted and flat). However, I suspect that 2 queen flat sheets work fine.