Kenny - the med cert isn't specific to the Paris marathon, it's a standard form used for all races in France. And, with slight changes in wording, there are similar forms for other sports such as cross country skiing or tri. If you get you doctor to sign the blank, make copies and write the name on afterwards, you can re use it for any other races you take part in, in France - it's valid for a year.
You can arrange for someone else to pick up your bib for you, but you need to arm them in advance with a copy of your Convocation (confirmation letter) which you'll get a few weeks in advance, medical certificate, copy of passport and a letter of authorisation. Worth arranging with someone if you think you might be late as there's no collection on the day. As Jimbob suggests, drop by the Paris thread and say hello.