With Tcl 8.4's vfs, all filesystem access except TclpCreateProcess (and a few related helper functions) is hooked into a lookup table. This means that all filesystem operations visible at the Tcl or C level can operate on a vfs, with the exception of 'exec' and 'open |cmd'.
It would be nice to be able to provide a more general mechanism to such command pipelines, especially given that stdout/stderr/stdin are already nicely abstracted as Tcl_Channels.
The stumbling block appears to be that all the 'pipe' related code doesn't actually use Tcl_Channels. Instead it uses a new 'TclFile' structure which is a lightweight wrapper around a native file-descriptor (and is slightly different on Windows vs Unix). Then there are a whole suite of new functions TclpOpenFile, TclpCloseFile, TclpMakeFile, TclpReleaseFile which are there just to operate on those structures. There is significant code-overlap between these TclFile-specific functions and the more general Tcl_Channel code.
Does anyone know any reason why all those 'TclFile' structures couldn't simply be removed and replaced with Tcl_Channels? If this were done, all of the 'exec' baggage and associated APIs could be made rather simpler and potentially exposed to the Tcl level through the tclvfs extension.