I've not been able to test it but this might work (of course, remove the logging once it does work);
Find the first element of the URI and lower it
set stringtolower [string tolower [findstr [HTTP::uri] "/" "0" "/"]]
log local0. "Lowered part of URI is: $stringtolower"
Extract the rest of the URI that we don't want to lower
set restofuri [findstr [HTTP::uri] $stringtolower "1"]
log local0. "Remaining URI is $restofuri"
Rebuild the URI and modify it before sending to the server
HTTP::uri /$stringtolower/$restofuri
log local0. "Rebuilt URI is /$stringtolower/$restofuri"