I discover it CAN be done in Sibelius. AND it will play back correctly with a couple of hidden metric modulations.
First input the notes as normal crotchet-triplets into a 4/4 bar followed by a normal quaver into a 1/4 bar, then hide the barline and delete the time sigs, so that it looks like a single bar.
[Turn on 'View hidden objects' if you like.]
Add some space at the start of the bar using a hidden grace note. Switch its playback off.
Click the R. button in space above the staff, and from the menu that appears select text|special text|time signatures. Then click the L. button where you want the time signature and enter '7' then '6'.
Now to make the final incomplete-triplet's bracket.
Select the final quaver in the bar and press 'L' to open the Lines menu. Scroll through it and select the bracket without a final hook. It's named 'bracket above (start)'.
Position it where you want it, and lengthen or shorten it by dragging the end of its line with the mouse. Now right click above the bracket and navigate to text|special text and click on 'tuplets'. Then left click and type "3". The 3 appears and you can drag it into position on the bracket line.
[Depending on your set up it's possible you may need to switch magnetic layout on or off.]
Finally use a new tempo for what was the 1/4 bar to make it last only 1/6, and another one to set it back to the preceding tempo. Hide these tempo changes. Sibelius will now play it correctly.
As Sibelius still sees the 1/4 bar as separate, the bar-numbering will need to be corrected.
[Turn off 'View hidden objects' if you like.]