I had considered something similar. If I had used solid plywood sheets I'm sure the bed platform would have been stronger but I used the pine boards so I can have "hatches" to access the space under the sofa. If this plan doesn't work out my next idea would be to weld a frame of angle iron that the bed platform rests in. Then just lift up the whole bed platform to access under the sofa.
The one hitch with this plan is I don't know how to weld. A friend who does has offered to let me use his equipment to learn. I'll consider this Plan B.