I see that HTML is disabled when you use the signature editor in Amiga.org.
So I guess since widgets use HTML code they will not work.
The weird thing is that the signature editor allows you to upload a signature image - but I can't see any way, once the image is uploaded, to insert it into one's signature. For instance, I've already uploaded a small icon graphic to accompany my signature, but it doesn't show up in the signature below.