Hi Sleep1,
I would replace the mattress itself and not try to fix it because they are just cheap low quaility coil mattresses which are just as big a problem as the metal bars underneath them. Adding a topper would only add the need to store it somewhere and you would probably be throwing good money after bad anyway. If you replace the mattress itself with something better you will probably eliminate or at least substantially reduce the feel of the bars (depending on the weight of the person) and you probably wouldn’t need a topper at all.
You can see some examples here of polyfoam, memory foam over polyfoam and latex over polyfoam sofabed mattresses (and their site here has the same pricing with a better return policy).
ADDED: Some of the better sources I’m aware of for sofabed, RV, or truck mattresses are listed in post #2 here.
If you did want the option for some extra “luxury” on top you could always have a 2" topper available as an add on for any guests that needed it but I would first replace the mattress.
Local foam shops could also put something very similar together and you would be in control of the quality of the foam you used…
Phoenix