Justoverclock I'm not sure what do you mean by "handle". New format of translations in Flarum is still just a string. And Weblate does not process these strings in any way, so it does not matter what kind of format for translations Flarum is using, as long they will be represented a string.
Right now the only problem is that Weblate always picks translations from the last release of extension. So if extension already released version for Flarum 1.0.0, new translations will be available in Weblate (and extension will be marked as incompatible on translation status page). If not, translations for old version will be available. So right now we have mix of translations for extensions targeting stable and non-stable Flarum.
And since Flarum core and official extensions did not released new versions, their translations in Weblate use non-ICU format. You can still translate new phrases on Weblate to match Flarum 1.0.0, but you need to ignore source displayed on Weblate and translate source directly from GitHub (see https://discuss.flarum.org/d/27235-extension-developers-flarum-10-qa-phase-update-period/17). But in general I suggest to wait to actual release of Flarum 1.0.0 - releasing extension targeting unreleased version of Flarum may bring more harm than good.