Low-energy dissociative electron attachment to BrCN and CBrCl3 over the temperature range 300-450 K is examined by measuring the velocity and angular distributions of negative ions produced through electron transfer in collisions with velocity-selected K(np) Rydberg atoms. The data are analyzed using a Monte Carlo collision code that models the detailed reaction dynamics. Measurements with BrCN indicate that electron capture leads to the creation of excited BrCN-* intermediates that dissociate with a mean lifetime tau similar to 20 ps to form CN- ions, the majority of the excess energy of reaction appearing in translation. No significant Br- production was observed over the present temperature range. Electron transfer to CBrCl3 is found to lead to the formation of both Cl- and Br-ions, the branching ratio depending markedly on temperature. At room temperature, Br- production is dominant, at elevated temperatures Cl- production dominates. The data show that Cl-formation is associated with two reaction channels: in the first the electron is captured directly into an antibonding orbital followed by immediate dissociation, in the second a longer-lived CBrCl3-* intermediate is formed with a lifetime greater than or similar to a few vibrational periods. The contribution from direct dissociation increases dramatically at the higher temperatures. Similar reaction channels are operative in the formation of Br- ions, but their relative strengths are not strongly temperature dependent. Possible reaction scenarios are discussed.