Would it be acceptable to ask for laughing gas just for fillings?

Sure: We use it all the time in our office for that purpose.

Laughing gas: You can always ask, but that doesn't mean that you will receive. Fillings are not likely to be painful enough to justify the small but present risk of administering nitrous oxide (laughing gas). You wouldn't want a doctor or dentist who would subject you to unnecessary risks.

Absolutely: In many instances, the nitrous oxide can be used in lieu of an injection. You may feel a bit of pain during the procedure, but the gas makes it tolerable and basically disassociates you from the procedure. When you're finished, no numbness !