Replicating Internal Redirect Behaviour For Redirecting To External Site In Htaccess

I have the following RewriteRule directive in my htaccess file on my localhost:-

Code:
RewriteRule ^([a-z-]+)/?$ test.aspx/app.nav/params.class.com${meta:$1}/whatever${meta:$1}
The rewrite rule works perfectly, since the revised URL is still preserved when after it is executed; I presume this is since my old URL is relative, and it's performing an internal rewrite.

However, when I make my old URL absolute (adding an external address to it), whenever I execute my new URL, I'm redirected to the external sites URL (which includes the same URL parameters as my internal 'test.aspx' page), like below:-

Code:
RewriteRule ^([a-z-]+)/?$ http://www.example.com/test.aspx/app.nav/params.class.com${meta:$1}/whatever${meta:$1}
Purely for demo purposes, upon execution, I'd like my new URL to be preserved in the address bar whilst, I'm redirected to the external site.

Is this possible to do - something like forcing the behaviour of an internal rewrite?

EDIT: I've done some investigating and people with similar problems have suggested using the Passthrough handler - although I'm slightly unclear as to what this does exactly, I gave it a go (adding '[PT]') at the end of RewriteRule, but unfortunately this didn't make any difference.

Is mod_proxy a possible solution?