Because the mouseover doesn’t observe spoilers, how about recommending that people not place spoilers in their OPs? One could put an interesting preamble in the OP and the detailed, spoilered bit in the second post.
It only shows the first bit so putting the spoiler a paragraph down will work too.
Just putting “Spoilers” in the title is not sufficient for the first post, since mouse-over can be inadvertent.
Most people are aware of the problems, and sometimes post twice to start such a thread. Or leave several blank lines.
And, if you find a thread that does have a spoiler in mouse-over, please REPORT it and a moderator will fix it.