window trim

I have to be honest here and say you may be over-angsting about something that's quite trivial.  The “no drilling, no nailing” by-law is a hammer to crack a walnut.  It's something the EC can use as a starting point when someone is gouging holes in their walls to the detriment of common property and their neighbours.

A pelmet is not a major construction and any holes you make fixing it aren't going to bring the wall or the window down.

A brief note to the EC telling them what you plan to do is probably all you need.  But, let's be honest, there are people installing wooden floors and entire kitchens without their ECs either being aware or, in many cases, caring.

I applaud you for trying to do the right thing, and maybe your circumstances are different, but really, if it were me, I would just drop them a line, invite them to come and have a look or a chat, give them a week to respond then get on with it.