The best test is to try it and decide for yourself. The engineers clearly did not intend this way to use the subwoofer but that by itself does not preclude any other use. It may be louder than it was downfiring but the sub can be adjusted to accomodate this change.
Front or Down Firing Sub
I know that there have been discussions of the merits of front vs down firing subs, and ported vs sealed. Mine is just a little different. I have a REL Storm III down firing ported sub which for placement reasons I would like to rest on its side (with proper feet support) and use front firing. Will this work or is there something in the design or the mechanics such as support of the magnet when sitting vertical that will cause problems.