Hm interesting. I am also postponing...though I just assumed they would resend waivers again next year if you remain an active applicant. I hadn't thought about sending thank you emails.
In that case Based on my emails, again as a fellow URM, Washington and Lee has been emailing me to highlight their diversity program. GGU in California also...
You can mention that you sent an earlier and that you wanted to follow up to confirm on your interview date. It's not something you should stress out too much over as follow up emails are common, as with admission counselors being busy