Hello,
As Shi Feng said, any URL redirect will not help you achieve what you want.
If the Web Dispatcher in use is from a recent release (I recommend using the Web Dispatcher 742), you can configure it for multiple systems, using the parameter "wdisp/system_X".
wdisp/system_<xx> - SAP Web Dispatcher - SAP Library
Then, you can create one external system, like "SID=EXT, EXTSRV=...".
The requests would still go through the Web Dispatcher. Thus, the URL will me masked as you want.
Cheers,
Isaías