Michael,
I don’t know the *reason* for this, but I imagine it has something to do with how many things need to parse the string before it gets assigned, kind of how escaping certain chars with the Regexp support in 1.4 requires things like “\\\\p”.
Anyway, the following code compiles without an error:
<bean:define id="var" value="\\\\d\\\\d" />
Can you try that, and see if you get the result in your taglib that you want? If not, I”ll kick it back to the dev for a reply.