I use a similar setup to you. Friction bar end shifter with X something sram 10 speed derailleur. I've routed the cable under the tape and it works fine, I know it's not indexing, but the derailleur returns very smoothly
I'm using the indexed right-hand one, but I could envisage taking the non-indexed front shifter as a spare if I was going on a month-long tour of Mongolia or something crazy. Is it not difficult to be accurate with a 10-speed cassette and no indexing?
Nah its easy. Only been on one big ride with that setup so far and it only took a couple of miles riding in London to get a feel for how much cable to pull/push in order to sit nicely in the correct gear. It was surprisingly easy.
I use a similar setup to you. Friction bar end shifter with X something sram 10 speed derailleur. I've routed the cable under the tape and it works fine, I know it's not indexing, but the derailleur returns very smoothly