There is a portable mini-split that recently came onto the market, and its form factor is similar to that of a window AC: https://www.gradientcomfort.com/

The only problems I can see are:

-Size/Price ratio: The tonnage (9,000 BTU) is relatively low for its price--it can only condition a ~375 sq foot space. Though the lack of install costs probably makes up for this unless you're doing a DIY job with a Mr. Cool Ductless HP.

-Incentives: Utilities may not want to spend energy efficiency incentive dollars on these, since someone could easily move and take the heat pump outside of the utility territory; which is not a problem with permanent HVAC installations. Might be covered by the IRA's rebates for Heat Pumps though.