I did successfully book a cruise with an online travel service, but had to cancel my reservation a month later for a family emergency. Not all online services do it, in fact, few do.
How did I find out who did? I just went through the whole reservation process (yes, every screen) and at the end you will know if they recognize the Share program by offering you either a GUAR or SHAR category. If you pick the SHAR, you will pay only the per person/double price and the cruise line will attempt to match you up with another like cruiser who is also travelling alone on the same itinerary. You might get a roomie, and you might not. In any case, you still pay only the pp/double price for one person.
has the HAL SHAR category .... but only after you go through all the screens to select a stateroom.
Hope this helps.