I am in the fairly advanced stages of developing an application for designing compositions on separate areas of say an A4 or US Letter Size sheet. GLabels is an example, but COMPO has a different look and feel, and I think a broader range of capabilities. It makes it straightforward to place different compositions on different areas, and you can also use it to generate arbitrary sized PNG graphics.
Another differentiating feature is that COMPO is written in the D programming language.
There's a lot to test - including the feasibility of the whole design. In a couple of weeks I should be in a position to post source code and some suitable build utility, and at that point I would welcome some volunteers to help.
If you are interested or willing to give it a try, please email me - email@example.com