So the fix turned out to be pretty simple once I had a good look at the situation and spent 10 minutes in the Canadian Tire plumbing section.
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These fittings-- 99 cents apiece, and as it turns out I really only needed 1-- are absolutely the perfect size. They're 3/4 x 1 inch adapter pieces for working with poly pipe. And their dimensions are absolutely perfect for this purpose: inner diameter 15mm, outer diameter 22mm.
First step: I cut two sections from the smaller side of the adapter. Each is 8mm long, which is just enough to prevent the front end of the LED bulb from touching the arc of the dome, while still allowing the bottom section of the radial LED emitters (if that's the word) to shine within the dome itself.
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I used JB Weld to glue these little "extension tubes" onto the back of the tach, on top of the clear plastic domes. I did this with the tach out of the instrument cluster so I could see what I was doing.
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Here it is re-assembled.
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The bulbs look like this, mounted in the standard tach lighting harness....
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It's hard to see, but here they are, inserted into the end of the new plastic extensions. They fit perfectly-- just a little bit of pressure, and the socket "pops" into the end of the pipe.
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Assembled, installed, and powered up, here's how it looks.
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As you can see, the tach lighting isn't quite the same colour as the other instrumentation lighting. Both types are sold as "warm white," but the colour temperature of the wedge-mount bulbs (used for most of the instruments) is 3000K while the bayonet-base tach lights are 2700K. I really didn't notice the difference while looking at the lights in the car. Maybe I was just too relieved to have them all working? Anyway, you can certainly see the difference here in these photos.
Best part of all: no hammer needed (thanks, Keith)!
