I'd imagine any registrar willing to extend a deal would want to know what 10,000 domains registered for 1 day will be used for.
As this could be used for malicious purposes such as using the domains to create catch all email address and mass password resetting accounts previously connected to that domain, be it domain accounts or elsewhere.
Sav allows you to grace delete up to 10% of your total domains with a full refund, perhaps you can find somebody with 100,000 domains at Sav which would grant them 10,000 free grace deletes that would be willing to help.
Dynadot' charges $3 per grace delete.
Epik used to charge less than $1 for domain tasting. I dont imagine they have enough domains still registered their registrar to support this anymore.
What's the cheapest no limit domain tasting you've been able to find?