If there is sufficient interest in adding this feature, I could easily implement it as a UserScript. The script could scan the WMD editor immediately before submission and convert
lp:whatever to the appropriate markdown.
Alternatively, I have developed a script that adds some buttons to the editing toolbar - one of which already provides an easy way to insert links to a PPA (including a link to detailed instructions on adding the PPA). Perhaps I could add another button that inserts the appropriate markdown for a Launchpad project.
Update: this has now been completed. Simply make sure that you are using Chrome(ium), Opera, or Firefox with GreaseMonkey and click this link (Opera users need to do a bit of extra work to install the script).
You should then have a toolbar button that looks like this when you hover your mouse over it:
Click "Insert Project Link" and enter the name of the project. The markdown will be generated and inserted into the editor.