It's "impossible" so far as I know. I believe that reference lines in a trellis chart are always calculated for the whole thing, not each sub chart. But see the first attached file for a workaround using expressions in a bar chart. Unfortunately, I can't see how this would work for a linear gauge. So I'm not thinking of any solution for your specific problem.
Well, unless you want to make a fake trellis chart out of multiple charts, in which case you can use regular old reference lines, but that's a lot of trouble, and I don't guarantee how robust it would be. See second attached file for that.
Apologies for not acknowedging your response earlier.
Problem is, I'm still trying to work out what it is you are doing!
Bear with me.
By way of background, the problem arose from client dissatisfaction with the range of symbols which were available in a Combo Chart - simple line symbols (one horizontal, one vertical) woudl resolve my issue.
(BTW, It is the subject if an Idea - might be worth a vote?)