It must be terrible trying to sell software development services if you work in Open Source. The restrictions of the GPL appear to make it almost impossible to demonstrate your previous work to prospective clients. I would like to offer the following advice, which is my attempt to distil what appears to be current best practice in dealing with this tricky situation.
Do not make it possible for your potential client to find any examples of code you have written. If you cannot avoid it completely, make it as difficult for them as possible. For example, if you have created a certain sort of plugin, just say "We have created a new Moodle block." Do not give any clues as to which block that might be. If the open source project provides a convenient place to upload and share your contributions, try to avoid making any code you have produced easy to find there. Even if you have been forced to share your code in these places, do not on any account provide your potential client with a link to the example of work that you are most proud of. It is fairer to let them search and find a representative sample of your work, if they are clever and patient enough to do so.
Your development staff are, or course, all faceless drones who are nothing to be proud of. Your potential client does not care who will actually be doing the work they are requesting. This is particularly important when the open source project has a strong community. It must not be possible to identify your staff in the recognised list of project contributors; nor should it be easy to discover how they have contributed to the on-going development of the project's code by looking in the project's issue tracker.
It is important is to dress your proposal up in meaningless marketing-speak. If your client cannot complete their buzzword bingo card while reading your document, they just won't hire you. Including a PowerPoint presentation with impressive diagrams and more platitudes about your company can create a particularly strong impression.