I only consider the following for cases such as yours, where compatibility with legacy code is useful or required and where other options are exausted. This is not an elegant approach, but it may be useful in special cases.
The XDBC protocol (which is what XCC uses) is supported natively on the exactly same app servers and ports which the REST API is exposed. You can see this on port 8000 in a default install. The server literally cannot tell a 'REST Application' and an 'XCC Application' apart except by the URI requested in the request (and in some cases additional headers like cookies). REST and XDBC are both HTTP based, and at the HTTP layer are very similar to the extent that they can share the same ports and configurations.
XDBC is 'passed through' the REST processing via the XML Rewriter. XDBC uses /eval and /invoke while REST uses /v1/eval and /vi/invoke. If you look at the default rewriter.xml for port 8000 you can see how the routing is made. While the XDBC protocol is not formally published its not difficult to 'reverse engineer' by looking at the XCC code (public java source) and the rewriter. For example its not difficult to construct URL and payload data to do a basic eval or invoke call. You should be able to replicate existing XCC.NET client behaviour exactly by using the /eval and /invoke endpoints (look for the xdbc attribute set in the rewriter.xml, this causes the request handling to use pure XDBC protocol and behaviour.
Another alternative, if you cannot solve the external variables problem is to write new 'REST Friendly' apis that then xdmp:invoke() on the legacy APIS passing in the appropriate namespaces. An option is to put the legacy code in an entirely seperate modules DB and then replicate the module URIs exactly with the new code. If you don't need to maintain co-existing versions then you modify the old code to remove the namespaces from the parameters or assign local variable aliases.
