Hi,
You can enter raw html into the signature, but it is limited to 255 chars, I'm not sure that is enough to fit your flash object unless maybe you load it in an iframe.
By default the signature is wrapped in an "UntrustedContent" control (we use NeatHtml to prevent XSS) so javascriipt or objects won't be allowed by default, but if you find your own user account on your site member list and click the gear icon to get to the manage user page, you will find a checkbox that says "Trusted". If you mark a user as Trusted then we render the signature without wrapping it in NeatHtml so javascript or objects are allowed.
Hope it helps,
Joe