I don't think that's possible either, you can create an existing component on the fly (from brs) but afaik you can't define a new one.  But, you can addFields after the fact to dynamically add any fields you need to your component.  so you might be able to create shell components (extend Group) and put any fields on them later.

I did try doing this with the video component, defining/creating it entirely from brightscript without a single XML reference. Now perhaps I did something wrong, but doing it this way resulted in a video that only played audio, didn't have any trickplay capability/bif files visible, and other issues.  The default video component when created in XML has about 40 separate children and features created with it by default that aren't present when you create it from brightscript only.