I had an HSG but wasn't able to try the same month (wish I was) due to IVF scheduling. I can hopefully be more realistic about the radiation though. I'm a cancer survivor and have had a lot of radiation exposure...almost 20 PET scans, CT scans and followup tests over the past 6-8years. I was so concerned about the cumulative effects on my egg quality that I emailed a fabulous site called the radiation health society where senior radiation techs answer your specific questions about radiation. They said that the "risk due to radiation" statement is a standard (particularly for legal reasons) statement and that everyone errs on that side (because they're scared if anything goes wrong and they didn't say it, you'll sue them). The same doctor pointed out that even with no radiation there's a 3-10% chance of some anomaly in any pregnancy and that in reality the effective risk from things like dental x-rays, chest x-rays, mammograms and pelvic snapshots (like HSGs) is exponentially small. Not zero of course, but then the risk to any potential embryo of our being 40+ far outweighs any risk from minor radiation.
It was more than enough to put my mind at ease and to be honest, after 2 years or so on this board, your centre seems to be the first to have ever expressed that concern (lots of HSGs here, including my own) so it seems an unusually tight precaution.
If I was you, I'd totally go for it.