KM, I never tried it, I have KM Bizhub Pro C65HC but don't like the idea of making plates on it for number of reasons:
-Toner has too much of wax, I am sure you are aware of UV Coating issue. We tried it - lays like a butter, shines awesome but comes right off with light touch of fingernail. That being said, i don't think toner layer is durable enough + it is kind of low melt.
-If God forbid that plate melts/gets stuck damages anything... Lets just say I'd rather buy one more HP5100

As far you being specific about purpose of printer with HP - simple salesman greed. They would say "Yes yes, by all means it is the better printer for you" even if you'd say that you plan on hammering down nails with it. I bet if you call back complaining that you can't make plates, in better case they will say that your user manual has a fineprint/disclaimer that they guarantee performance only with approved media, in worse case they will void your warranty because you used it for something that printer was not really meant to be used for, person on other end of the wire may not differ poly plate from plywood, especially popular these days overseas tech support and pretty much everything else.
I can work on HP, not only I am a business equipment service engineer, I own printshop and used HP 5100 for years so I may claim that I know what you need. As far as Xante, it is relatively new to me plus Xante hold everything to themselves, there are very little aftermarket alternatives to it. If we/you go with Xante, I can do the labor but if we need parts, there is a good chance that we'd have to go to xante and pay full price. I have done some research and turned out to be that what lays inside the Xante Platemaker is B/W ancient printengine made my Fuji. By measurements of paper printing world it is POS - bulky, slow, expensive but capability to take 13" wide media and running really hot found it's purpose at Xante. Too bad it was dropped by everyone else long time ago so Xante came on top as a winner. I think PressTek bought Xante, to add more bad news to the story. I also have JetPlate system which is defunct bankrupt long time ago but came with Harlequin RIP and Epson 4880 equipped to run aluminum plates. I was planning on to convert it to something called EPSON CTP these days, only one thing missing is baking oven. If I can quickly figure it out, I can offer you more choices. Our Xante PM-4 has less than 200 plates made so it would be a nice buy plus since it is on my floor, I can offer you a sample plate.
Lets get back to this in a day or two - I will Email you with, hopefully, good news.
Roman.