What health-based guidance values have been set by regulatory agencies for thallium, other than the USEPA reference dose for drinking water (2 micrograms/litre)?

You can find the derivation of RfDs used by USEPA in: U.S. EPA. Provisional Peer-Reviewed Toxicity Values for Thallium (Soluble Salts). U.S. Environmental Protection Agency, Washington, DC, EPA/690/R-12/026F, 2012.

That may help you decide if you can apply it to your assessment.

You can also find screening levels developed for soil and tap water using these values at Regional Screening Levels (RSLs) - Generic Tables | US EPA.

The US drinking water quality standards stipulate the content of thallium: thallium is an inorganic pollutant, with a maximum allowable contamination level of 0.002 milligrams per liter in tap water, while the public health target is 0.0005 milligrams per liter. If the maximum allowable contamination level is exceeded, it will cause hair loss, changes in blood composition, and affect health problems such as kidneys, intestines, or liver. Thallium, as a pollutant in water, commonly comes from leakage from ore processing stations and emissions from electronics, glass, and pharmaceutical factories.

The United States Permissible Exposure Limit (PEL), the ACGIH Threshold Limit Value and the California OSHA PEL are all 0.1 mg/m3 as an 8 hour Time Weighted Average.