I sent a note to CATSA several year ago, and inquired about that very topic. In particular, I was eager to find out what potential there was for DIY pedals to elicit alarm bells. Their response (which may have been predicated on different notewrthy events, and different technology available at the time) on the matter was reasonable, and emphasized that the least problematic approach was to simply ship the pedals ahead of time. If not, simply declare that the item is there.
I could post their verbatim response, but the free wi-fi at the hotel we're staying at in Glasgow seems to be blocking my use of the search function in the forum where I originally posted the note-exchange. I'll see if the next place we stay at lets me do it.