What are the maximum organic matter and salinity levels that limit TDR use? Can I just adjust my calibration curve to account for these effects?
Organic matter affects TDR readings quite differently than salinity in the soil. In general organic matter affects TDR readings only when organic matter is a dominant component of the soil matrix, such as for soils classified as organic. The correction for organic soils is to use an alternate calibration curve. The following reference shows calibration information used by a colleague working in peatlands.
Lapen, D. R., Price, J. S. and Gilbert, R. 2000. Soil water storage dynamics in peatlands with shallow water tables. Can. J. Soil Sci. 80: 43–52.

In the following reference we have investigated how the soil electrical conductivity or salinity affects the use of TDR water content measurements.
Topp, G.C., S.J. Zegelin and I. White. 2000. Impacts of the real and imaginary components of relative permittivity on TDR measurements in soils. Soil Sci. Soc. Amer. J. 64:1244-1252.