From the documentation, I see that I can specify the target "bank" (LED1_A/LED2_A/LED3_A or LED1_B/LED2_B/LED3_B) and "trim" (nominal, +9%, or +10%) in the MEASCONFIGx parameters for each channel, but I haven't been able to locate any documentation about how the "bank" parameters that are being pointed to are configured to specify a current value.
There should be "30 current settings between 5.6mA and 360mA", but the "bank" parameters aren't defined, and every piece of example code I've come across is setting all the "bank" parameters to 0x3F.
Refer to Table 8.8 "Typical LED Current Vs. LED Code) in si115x datasheet, re. 1.3, page 49.
and specify per the led param (a value of 3e, for LED 1A is shown)
retval += Si115xParamSet( si115x_handle, PARAM_LED1_A, 0x3e);
Table 8.8 in the datasheet lists all the available LED code.