I too wish I know the correct answer. A standard part of my booking process is to always ask about special needs, whatever they may be. There is a fixed number of wheelchair accessible cabins but many people confined to a wheelchair book regular cabins regularly. Some because the categories they want have no more openings and some for other reasons. I don't pry too much but I do notify the cruiseline if special needs are present (unless the client asks me not to, which happens more than you'd imagine). Most of the lines have a form they require whether or not an accessible cabin is chosen. There must be a reason for such notification requests.
Where I work it has become a large enough segment of the cruising world we have a seperate Dept trained just for special needs bookings.
I have one big pet peave in this area. You'd be shocked how many clients with no such need for a wheelchair accessible cabin have discovered they have larger bathroom (for turning radius) and are often larger than other staterooms in the same category. We get calls regularly asking for these cabins by able bodied clients. Much like you often see abuses of handicapped parking spaces. Some lines are finally cracking down and requiring the forms before they'll release these cabins but some lines have a first come first served basis for booking these.
Please leave these for those who need them.