I think a broader issue related is that “transfer” in the context of Harberger Tax should be different from that in ERC721. ERC721 is designed for private ownership, while Harberger Tax is designed for partial common ownership.
Ownership under Harberger Tax corresponds to tax obligation, so transferring token is also transferring tax obligation. This can be used as a form of attack, where the attacker set the price very high and then transfer it to the victim. (In the case where tax is prepaid/deposited and allocated for each token, it can be mitigated by clearing deposits of all tokens I do not own, but it can still cause unwanted consequences)
In our case, we automatically set the token price to 0 when a transfer is triggered. “Transfer” thus still follows the continuous auction logic of Harberger Tax, where the owner set the price to 0 then the receiver gets it for free.