I don't think it's possible to evade tax cos once I submitted my rental application for whole unit to HDB, Tax Department automatically changed my flat from owner occupied to non owner and charged me higher tax. When I moved back after the TA ended, I had to inform Tax Department and they reinstated the owner occupied status and refunded me the balance tax as they collected 1 year in advance.
The only way to avoid tax is for owners to rent out their units without HDB approval, that is, not submitting applications to HDB which I think it's rare cos FT need to register their residence with their employers and MOM. Don't think FT will rent if they can't register the address. Probably if tenants are Singaporeans and owners want to take the risk to save on the tax. However if tenants want to bao tou owners later, sure can if owners are nasty.