I solved this issue in the 727 last night.
The 727 had 2 trims, a fast one and a slow one.
The fast one is for Takeoff and landing.
The slow trim is called "Cruise Trim" to make adjustments at cruise.
The fast one gets the standard trim assignment on your yoke.
The Cruise Trim switch is located on the Throttle Console and can be manipulated with the mouse.
But who wants to do that with the mouse? We gotta get the mouse outta there.
I was working on another project involving LINDA, LUA, MilViz's 737-200, PMDG's 737NGX and a custom MCP using Arduino and Link2FS.
In my searches I stumbled upon a macro tutorial for FSUIPC.
I believe this fix requires FSUIPC to be registered.
Anyway using LINDA I found the variable name for the Cruise Trim and its parameters.
The variable name is StabCruizeTrimSw2_click.
It uses 3 Parameter values 1,0,-1.
1 is Trim up.
0 is released.
-1 is Trim down.
To make it work with a key or button assignment we need to make a macro file (.MCRO).
Open your favorite text editor and put this in it.
Now save the file as something.MCRO.
I called mine 1cs727.MCRO. The 1 will make it be at the top of the list in FSUIPC.
Place this file in your FSX "Modules" folder/directory.
Start the sim and go into FSUIPC Buttons + Switches Tab.
Check box Profile Specific
Press the Joystick/Yoke/Throttle Quadrant button that you want to make Cruise Trim pitch up.
(NOTE:I use the far right switch on the CH Throttle Quadrant. Down button for pitch up, Up button for pitch down.)
Check the "Select for FS control" box.
In the drop down window named "Control sent when button pressed" you should see your macro command.
(Mine looks like 1cs727:L:StabCruizeTrimSw2 Click Set)
In the Parameter box right below, put a one, 1. For trim up.
Check the "Control to Repeat While Held", box.
In the "Control sent when button released" drop down, select the macro name again.
In it's Parameter box put a zero, 0. To release.
Now repeat the Assignment Process for Trim Down using the same macro command but change the Parameter to -1.
I hope this is clear, I took some benadryl and I'm loopy.