Hi Chris,
at the time only from frsky sensors.
Getting rpm from the ardupilot stack would require a library change on the ardupilot side!
If you could test all this it would be great!
I’ve been looking for flexible solution for quite some time now 
Sensor numbering is
1 - 2
3 - 4
5 - 6
Sensors 1 and 2 have fixed font (small)
The configuration file is quite easy to deal with, for instance the krpm config section is
-- Sensor 3
[3] = {
"ENG", -- label is ignored on taranis
"RPM", -- OpenTX sensor name
1, -- precision: number of decimals 0,1,2
"krpm", -- label for unit of measure, on taranis is appended to the value
0.001, -- multiplier if < 1 than divides
"+", -- "+" track max values, "-" track min values with
2, -- font size 1=small, 2=big
110000, -- warning level
120000, -- critical value
},
You can use the same sensor twice, one for engine rpm and one for head rpm by using different multipliers.
Alex