Just treat it as normal really... Think of the reverse proxy as really just load balancing over one backend... Since the number of backends is pretty much irrelevant (Assuming you can do persistence, and since you say web servers you can do active cookies), you can forget that bit.
The more important question is what functionality do they want to achieve from using the 'reverse proxy'?
Client Authentication? No problem, it's a separate license module though I think...
Authorisation? Hmm... That's a good question... I'm not sure how you'd do something like lookup arbitrary attribute sin an external LDAP (Or similar) directory for authorisation decisions...
You may still need some external web host to do the authorisation lookups for you... And then scrape the results from the reply and put them in persistence records for later... Hmmm... I wonder if iRules are flexible enough to do a separate lookup itself against an auth service then direct the user request based on the results of that... Anyone?
H