yes, it is possible to do what you want. Hard to know exactly which rules to use, but something like this is pretty close:
# rewrite all URLs with OpenIn query string param, to specify "Browser"
RewriteRule ^/([^\?]+)?(OpenIn) ^/$1?OpenIn=Browser [L]
# Rewrite other Urls (that lack OpenIn) to specify OpenIn=Browser
RewriteRule ^/([^\?]+)?? ^/$1?OpenIn=Browser [L,QSA]
The edge cases not handled: when you have a URL with multiple query string params, one of which is OpenIn and the others are not, you'll need to deal with that, possibly with a separate rule. I'm not sure this is even possible, and not sure what you'd want
to do about this sort of incoming URL.
Also you may have other cases you want to cover. The only way to know is to test it.
Perhaps I'm doing something wrong...
I found the TesrtDriver gizmo and took your first example and in the SampleUrls.text file I put in
In the iirf.ini file I have your example of -
RewriteRule ^/([^\?]+)?(OpenIn) ^/$1?OpenIn=Browser
Ran it and got the following output:
TestDriver: linked with 'Ionic ISAPI Rewriting Filter (IIRF) 126.96.36.199 x64 RELEA
TestDriver: The IIRF library was built on 'Apr 18 2011 01:41:16'
Trying to read config at '.\Iirf.ini'
done reading new config
Retrieving server variable that is not supported by TestDriver (SCRIPT_NAME)
NO REWRITE '/FormServer.aspx?OpenIn=Car' ==> --
1 Errors in 1 Total Trials
I'll keep futzing with it, but if you have any ideas, it would be great.