The IKEA Markus is a nice chair, but it's not immediately suitable for mounting a joystick and throttle using VESA brackets. The arm structure is too thin and curves the wrong way.
My solution was to use a pair of rubber wedge doorstops to change the angle at which the bracket would clamp to the chair arm, and increase the thickness enough for the bracket to get a grip. This is all that is needed for this hack. The allen key and the spanner were included with the Pro Signal bracket. (I had already completed the mount for the throttle before I remembered to take pictures of the process.)
Close-up of the bracket as it comes out of the box.
The caps over the joint mechanisms are removable by hand (with difficulty) or using a flat screwdriver to pry them off. For this hack, I set them aside instead of replacing them after adjusting the joints.
The reason for setting aside the joint caps is shown here: the cap blocks the outer joint from turning a full 90 degrees.
With the cap removed, the joint can turn beyond 90 degrees, which gives needed leeway when mounting the bracket to the chair.
The bracket is now clamped to the chair, using the doorstops to change the angle and thicken the arm. As one can see, the bracket isn't quite horizontal, but it is close enough that the outer hinge can be turned to the vertical.
Close-up of the bracket platform.
I couldn't find dual-lock tape, but I was able to get industrial-strength Velcro. The package contained a pair of sets, 2" by 4" (5cm x 10cm); cutting a strip in half lengthwise made for a good fit, and as a bonus, one package is good enough for both arms of the chair that way.
The completed hack: the X52 Pro joystick and throttle are securely and comfortably mounted to an IKEA Markus chair. The wire connecting the joystick to the throttle can be tucked out of the way by securing it to the chair arms below the bracket with your method of choice (I used Velcro wire-ties that I had from another project).