ReWrite in web.config to go between mobile and desktop sites - mobile

Question

I am finishing up a new mobile website to compliment the desktop version. Currently I have a rewrite rule in the web.config file on my desktop site that looks like so:
<system.webServer>
<rewrite>
<rules>
<rule name="MobileRedirect" patternSyntax="ECMAScript" stopProcessing="true">
<match url=".*" ignoreCase="true" />
<conditions logicalGrouping="MatchAll">
<add input="{HTTP_COOKIE}" pattern="nomobile" ignoreCase="true" negate="true" />
<add input="{HTTP_USER_AGENT}" pattern="android|blackberry|googlebot-mobile|iemobile|iphone|ipod|opera mobile|palmos|webos" />
</conditions>
<action type="Redirect" url="http://m.mysite.com" appendQueryString="false" redirectType="Found" />
</rule>
</rules>
</rewrite>
This works perfect if I never want the user to be able to use the desktop site while on a mobile device, but that is not always the case. Some links on the mobile site DO link back into the desktop version. I have a link on the footer of the mobile site to "view full site" as well.
So my question IS: how to properly handle cookie setting on that link and then detecting in the web.config and NOT redirecting to the mobile version IF it exists.... I have a conditional check in the web.config for http_cookie "nomobile", but I don't think it is properly working. Do I just send in a querystring value from the mobile and check that in the global.asax file or does that not work because the web.config runs first?
The desktop is a C# MVC4 site on IIS 7.5 if any of that helps, and the mobile site is a simply jquery mobile site.
Thank you!
EDIT:
I have tried checking the querystring in the global.asax file (code below) but it seems that "Request is not available in this context".
// create and set cookie if ?nomobile detected
string forcedesktop = HttpContext.Current.Request["nomobile"];
if(forcedesktop != null){
HttpCookie nomobile = new HttpCookie("nomobile");
Request.Cookies.Add(nomobile);
}
}