generating one tags database for multiple sub-projects

[vg23]

Hello,
I just started using Global, and it seems like a really great tool which 
would be very useful for my purposes if it can be used in the following 
situation:
In the code that I have to work with, there are a number of subprojects 
that are located in separate source trees, an arragement of the 
following form:
commonroot/project1/devel/src/(a source tree here)
commonroot/project2/devel/src/(another tree)
commonroot/library3/src/(a yet another tree)
and so forth. 
The code in any given tree might refer to code in another tree.
It would be really great if it were possible to get gtags/htags to 
generate a single database for the whole project, as opposed to having a 
separate set of G* files in each source tree. 
Unfortunately, I do not have the permission to write into commonroot!
(furthermore, even if i did--there are other (large) projects in the 
commonroot that I don;t need to be included in the tags database). I tried 
setting up another directory containing symlinks to the appropriate 
project source trees, but running gtags from there produced an small file 
that had no  symbols in it (I guess it doesn't follow symlinks?).
Any advice on getting around this problem would be greatly appreciated!
Thanks in advance!
Regards,
        Vadim G.