I have the exact same question for the L-Lounge in my International Serenity 28W -- so am following this thread. I suspect though for me it would be alot tougher, as my water heater and charger/converter are smack directly underneath the short leg of the L couch.
Someone posted into the Facebook Airstream Addicts forum a link to a sofa able to be installed on a flat platform, such as is my L-Lounge; that I don't loose the under storage. I am trying to locate it now - the images appeared to show a modernized gaucho'esque couch/bed option; but with weird amenities like USB ports built into the frame, etc. I'll try and find that link and post it into the thread for your viewing.
Found it -- Lisa Wemple on Facebook posted to the Addicts forums a couple of posts revolving around a sofa she used during a rebuild -- a Serta Palermo. If you're a member of the group on FB, do a group search for Palermo and it will return the post involved. if you want nothing to do with the group, or FB, I am copying an external link to the product via a third part company, below.
My thoughts is it would be relatively easy to remove the current base and re-attach it to a flatboard base that is the L-Lounge, either in a permanently fixed fashion that allows for sectional recline, or on heavy duty lock/slide-out mounts to allow for a fuller futon/bed experience for guests or an afternoon nap near the door.
External link -