The issue is about the linear RCS, which the Velcro upper stages don't have. I believe [ame="http://www.orbithangar.com/searchid.php?ID=2872"]Ariane 6[/ame] (as in the fictional, super-heavy launcher add-on) has a tug with linear RCS, but that's too big for EELVs as well.
It's likely that I'll make a tug for my Hatsunese space station project, but sorry, it's not going to be released right now.
My main issue is that a tug attached to the back of the module won't have the RCS thrusters at the center of mass, so there will be a torque when using the linear thrusters, unless the vessel is kept in Kill Rot mode (using up propellant to counteract the rotation), or the RCS thrusters are on booms.
(this image is from a
.pdf about future derivatives of JAXA's HTV)
The problem with the booms is how to deal with the different sizes/lengths of modules.
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The other solution to the center of mass problem is to have
a tug that surrounds the module, but there might need to be either a wider fairing or narrower modules. Also, speaking in in-simulation terms, there needs to be a "docking port" or "attachment point" in the center of the module. (there is a problem with attachment points, though, and that is that the mass of the child vessel attached to the parent isn't really taken into account)