- The One True Sysadmin will look through all the logs and find your server and block it and you'll have to pick another one next time

- The One True A(G)I will find your domain automagically and block it and you'll have to pick another one

...In all seriousness several MB of data to a DNS server will get you bitten at least once because someone somewhere is actually doing the job they're paid to do. But it'll probably be an exception.

As another replier noted, you might evade detection. But the reason I mentioned it is that captive portals (before authenticating) may not allow you at all to do DNS queries... except, for their own whitelisted domains. Depending on how they do it and how they wrote their rules, if they allow example.com you could potentially query example.com.mydomain.com