Claire Perney gave me the number of the institute missionaries in Paris. I called them and asked them whether they could help me find a family to stay with (to help minimise my costs for accommodation) and they said they'd ask at institute—they were just about to go down to class—and let me know. I left my phone number and email address... let's see what happens.
Accommodation is probably the most expensive item, especially if I consider hotels (youth hostels are a bit cheaper, though I'd have to buy a youth hostel association membership first); then my next big worry is travel but maybe we can manage that. And if I'm really lucky, I'll get a talk accepted so that I won't have to worry about the conference fee, either. But I wanted to see what my chances for accommodation are before I start worrying about travel or conference fee.
*Phew*, it was a bit nerve-wracking to have to speak French on the phone to the secretary at the institute office; fortunately the missionaries themselves spoke English (at least the sister to whom I was transferred).