Build Entity Form Url

a Microsoft CRM 2011 question. Given the following parameters:

• An entity schema name (i.e. opportunity, new_customentity)
• An entity guid
• A CRM organization base url (i.e. http://myhost.com/ORGNAME/)

How can I build an url to point to the main (or "default") form of that record?

I can tell a pattern in form urls, but some querystring params are pretty obscure and I wouldn't know how to reuse them for any other kind of entity. In addition, version 4.0 had specific URLs (/sfa/, etc) for system entities. It doesn't seem to be the case anymore.

Any help would be appreciated - thanks.

(additional tags welcome, I cannot add them.)