Looks like the current Sibelius team failed to address this feature request.
As what I tested, the current version of Sibelius (8.4.1) still does not allow me to place a sounding fermata mark above a measure splitter.
I am wondering whether you guys addressed this in Dorico.
I very much hope it will be implemented before the first public release, but it remains to be seen! If not, it will be added as soon as possible after the initial release.