ISAPI Rewrite Filter - Alternatives To RewriteMap

we're using IIS 5.0 which doesn't have any flexible support for rewriting URLs which include parameters (AFAIK, anyway). We're currently testing an ISAPI Rewrite Plugin, which emulates the syntax and behaviour of mod_rewrite, which is why I felt this forum category to be the most appropriate for this post.

Our URLs at the moment look like:-

[url]where 'hg' is an instance of a key. In this particular example, we wish to rewrite our URL to read [url]homegarden' where 'homegarden' is a value of that respective key.

Since the value 'homegarden' isn't present in our current URL, we need some way to associate this value with it's relevant key, by mapping one to another. The ideal solution in this case would be to use RewriteMap, however the ISAPI plugin doesn't support this directive.

One other solution, that I've seen here is to use the 'homegarden' value as the key instead. However, due to the lack of flexibility of our system, this isn't possible.

Another method of achieving this is to hard-code a RewriteRule for every instance of a key, but this solution has obvious performance drawbacks (we have around 400 categories)

So, at the moment I'm out of ideas, which is why I thought I'd try and rack your brains for suggestions. An ideal scenario would be to use something like RewriteCond (which is supported by the plugin) to attach a file which would contain the key/value map.

The current supported directives are :-

RewriteRule
RedirectRule
RewriteHeader
RewriteCond
RewriteLog
RewriteLogLevel
RewriteFilterPriority
IterationLimit
MaxMatchCount
StrictParsing

Even if it's not possible using one of the above, maybe someone can point me to solution involving a method other than using this particular plugin?