My understanding and personal use of the SW600 switches is they only talk to
rear deraillers, (brifter was not needed), so your approach would not work with a 2 or 3 chainwheel
system. My inference about Di2 is that the switches in the brifters and SW600
send specific codes that the deraillers respond to, ie there are tiny coding
chips within the switches that generate these codes and debounce the
mechanical switch. My reasoning is based on the fact that shimano does
not label the ports on their connectors, any port can be used for any cable.
If you could adapt one of the tri bike switches that plugs into the end of the
tribar you could then control the front derailler, but the SW600 won't do FDs.
This would require some ingenuity.
Some enterprising cyclist back in the day of DA only Di2 did describe a chip
swap, IIRC to adapt for tri use, but this is not for the faint hearted or light of purse.
OP may have discovered Shimano is WAY behind on shipping ultegra Di2
hydraulic levers, which seem only to be available to certain large manufacturers
and been greatly delayed to the retail channel.
A bit of further googling suggests:
https://www.competitivecyclist.com/s...switch-sw-r610
is the switch you would need to control the front derailler. I have not used this switch, which unlike the
SW600 comes in R/L pairs. My use of the SW600, it works fine controlling the RD on my recumbent,
which does not have an FD as such, and no brifter needed. Dunnoh about the SW R610.
Further reading, SW610 only works with the RD, not for FD, one switch downshifts RD, other switch upshifts RD.
http://si.shimano.com/pdfs/um/UM-7FL0A-000-00-ENG.pdf