It's a bit long winded, but might be possible if you have a Wii U.
I'd look up the software, but I'm at work. I'm sure it'd be easy to find.
There was an application on PC that came out in like 2006 with the release of Wii that let you hex edit / modify mii's and transfer them to a Wiimote via bluetooth.
I recall this allowing you to set Gold trousers on a mii.
You then transferred it from the Wiimote to the wii using the original Mii maker app on the Wii.
I think you might be able to transfer this from the vWii app to the Wii U Mii maker via one of the standard import options and then from there transfer it from the Wii U Mii app to the 3DS.