I wrote to Mark Mueller of Incra Tools and ask his advise on the subject of this thread and what he wrote back can be read below.
Your miter gauge will work fine for compound miters if you approach it the
right way. I would recommend not tilting the saw blade - as you've noticed,
the angles get strange, it's all trial and error, and setups are difficult
to repeat later.
The easiest way to approach this is to keep the blade vertical but angle the
lumber off the sled (your Miter Express). If you want a 4-sided frame with
a 10 degree compounding angle, the miter gauge would be set for the normal
45 degrees and the lumber would be angled up 10 degrees. Trim carpenters
always use a variation on method on miter saws when installing crown
moulding and absolutely never tilt their saw's blade.
There are a variety of ways to angle the lumber off the sled, but for the
couple of times I've done this, I think the safest bet is to tilt the table
on a band saw 10 degrees and rip a wedge off of a straight 2x4" or other
scrap (the wedge should be slightly wider than the width of the moulding).
After the miter angle is set, the wedge is stuck onto the sled platform with
double side tape (cloth-based carpet tape or paper-based lathe mounting
tape, not the thick foamy stuff). Putting the thick edge of the wedge
against the fence allows cutting the moulding good-side-up to eliminate
To do all of this without switching the miter gauge to the opposite side of
the blade, you'll need to cut in both open and closed positions on the gauge
- one end of the moulding cut with the outboard end of the fence pulled 45
degrees toward the front of the saw ("opening" the miter angle), and the
other end of the moulding cut in the more common "closed" position with the
outboard end of the fence pushed toward the rear of the saw.
This isn't more dangerous than other mitering work, but it has a different
feel so work carefully.