I just spent way too much time on this so I thought I'd share my solution.
I was unable to get the output modifier to evaluate the contents of my placeholders from MIGX. I tried caching the placeholders in various combination to no effect.
[[!getImageList?
&tvname=`grid-content`
&tpl=`@CODE:
<div class="oneThird grid [[+yellow-background]]">
[[+grid-copy]]
[[+grid-video:notempty=`
<div class="video-wrapper">
<div class="video-container">[[+grid-video]]</div>
</div>
`]]
</div>
`]]
Finally I moved the @CODE: into a chunk and called it like this:
[[!getImageList?
&tvname=`grid-content`
&tpl=`content-grid-migx-tpl`]]
Now my output modifiers are working correctly.