You're in luck. So far as I know ioFTPD is the only FTP server to really solve this problem...
First make sure you have the feature enabled... 'Dynamic_DNS_Lookup' under [Network] should be set to 'ALWAYS' (KNOCKED is more restrictive and a lot trickier so just use ALWAYS).
After that the form you used should be all that is required - ':ident@blablabla.dyndns.org'. If no "static" hostmask matches the user it will resolve at login time any hostmask starting with ":" to get the current IP and check that.
I do wonder a bit at the 'hostmask *@1.2.#.# not added' bit. That isn't an ioFTPD message but must be the result of some script running. It's possible that you have a 3rd party script trying to help out and automatically add some hostmasks to users and that is breaking the internal feature. Check to see if there is a PRE/POST event on the USER or PASS commands...
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