Many thanks for this pointer @KeerthiRocks! Since my SCB is on Port 1, I put the following right at the start of my main routine:
(*(reg32*)(CYREG_GPIO_PRT1_PC)) |= 0x1000000;
With this there I measure a threshold of 1.6V while with it commented out the threshold is 2.1V. This is measured on another P1 pin because I have not got a suitable way of measuring it on the actual SCB RX pin, but I assume that must go through the same input circuit otherwise why would PSOC Creator object to having the Pin component properties set to LVTTL? To be thorough though maybe I need to set up a test with a potentiometer between the USB converter and the PSOC RX pin.
As feedback, it would be better if PSOC Creator represented the SCB TX and RX as ordinary Pin components on the schematic (a composite component) so their properties could be set directly. This would also allow external components connected to these pins to be represented on the schematic in the same way as for other Pin components.