I think your mural sounds wonderful. I'm sure your students will remember the experience for the rest of their lives...and come back at reunions to see if it is still there! I'm like you...I want the kids to do as much as possible - it's hard sometimes when you "compete" with other schools/teachers who do things that really wow the public, but weren't really strong art experiences for students. I guess that's when you just bite it, stick to your guns, and know you're doing the right thing!
About the issue of pulling kids out of "specials" to finish work - I had to raise a stink one year about some teachers who were chronic at this. If it's just once - so the student gets a wake up call about getting work done -then I say yes.