Got an answer from Regitra today: Temporary/transit plates (red letters on white background) can be assigned to anyone, including persons who cannot provide a proof of residence or regular abode in Lithuania. They have a validity of one month.
Regular (black-on-white) plates require a registered residence in Lithuania. There is a way to get a six-month registration without being officially registered as a resident; this, however, requires some proof of abode such as a rental contract. Bottom line: regular plates are not available to non-residents.
Either way, registering a vehicle requires the following:
- Proof of insurance
- Proof of TA (techninė apžiūra – vehicle inspection), except when getting transit plates (red plates) for the first time
- Proof of vehicle identity
- ID of the person registering the vehicle
- Previous registration certificate
- Proof of ownership (e.g. sales contract), unless the previous registration was already in the requester's name
- Previous license plates, unless they were lost or returned to the issuing authority