Your BookPlate design is lovely, Calion...and while I don't know if there is an option to pre-number a Custom BookPlate because I've not yet had an opportunity to download the Adobe Flash Player needed to try it, the Supply Store does have a tab for creating your own Label under the BookPlate Category...have you tried it? https://www.bookcrossing.com/mybookplates
You can order prenumbered labels from the Supply Store, although the current custom bookplate templates don't support the label style in your (handsome!) example.
You may already know about the wings option to allocate sets of prenum BCIDs from your profile page's drop-down listbox - there's an option for "Pre-numbered list" at the end of the options, which will show you any as-yet-unactivated prenums already allocated to you. It also has the "request" option that lets you allocate 10 or 100 additional prenums, which you can then write on your own labels. If you have a label template that allows form-filling, you might be able to easily generate printed labels with the allocated BCIDs on them, but you would have to be careful to get the numbers right. (I don't know if there are label-creation sites out there that would accept a list of numbers to add to the labels; I'd imagine it would be possible, but I have no idea how pricey it would be.)
One MAJOR note: if you start messing around with the prenumbered-list, be very, very careful. The list shows all as-yet-unactivated prenums, which will include any Supply Store custom bookplates that you ordered with prenums, as well as the free downloadable prenum-labels. And if you ever used a prenum label on a BC book and released the book without activating the prenum yourself, that prenum will stay on your list unless and until someone else activates it. All this means that you can't assume that the numbers on the list are available for use on new labels; you could wind up with duplicate BCIDs out there!
Extra bonus weird note: in the past, I've noticed that some of my Supply Store custom-bookplate orders for prenumbered labels wound up adding more than one set of prenums to my list - perhaps there's a trial-and-error step to get the formatting right or something. But the result was that I'd receive a lovely set of labels with prenum BCIDs on them, and would also have another set (or two) of prenums that were allocated to me but not present on any labels. So I'd make careful note of which sets were which, and use the bonus BCIDs on other labels - being very, very cautious about not duplicating any. Not sure how often this happens, but if you do keep track of your prenums, and if you always activate them before releasing your books, you should be able to tell whether there are any BCIDs allocated to you that aren't on any of your labels yet.