I only have CS3 at the moment, and have sent a file to someone using CS5 (or CS6). They do not have one on the fonts I've used in the file on their system. They've attempted to return the file after working on it to me, but we run into the missing plug-ins issue (perhaps there are other incompatibility issues as well). They've attempted to re-send as an idml file, but I understand that in order to open this in CS3, one must first go through CS4 (neither of have), and change it to yet another file extension.
Is there a workaround for this?
No. And that's one awful workflow that is filled with trouble.
Download the CS6 trial and use that.
I'm on an older machine which may not be be up to spec for running CS6 and have several projects to complete in CS3 this month before coping with a learning curve for the upgrade. I take it you strongly advise against going any route other than the one suggested?
It’s a very dangerous workflow doing this for round trip and no, there is no way at all to go from CS5 and above to CS3. CS4 has to be in the mix.
If you want to stay in this business you’re going to need to buy a new computer. You can save a ton of money by moving to Creative Cloud and if you do so by August 31, you can get intro pricing of $29.99/month for the first year.