Hmm... noting 6/8 as 2/4 is easy. Write each as a quarter note except when you have the triplet. Finale does that easily. Sousa marches and Bach call for that. You can have a triplet with the bracket hidden.

Writing as dotted quarters in 2/4 makes no sense—that's 6/8. Focus on the results, not on how you think it should work.

Write it in the time signature that works. Hide the real time signature. You can add 2/4 as a graphic. 

How you you convince a player that each beat in a 2/4 bar should be subdivided to 9 is another issue.

Many thanks. You have solved my problem for me. I will admit, I didn't think of that approach. Now that I know how to do it, I have made a 1 measure demo in .pdf format, which I could post if you are interested, but I don't know how to put the file in the posting. If you let me know how to do that, I can show you the demo.
Cheers, Rick

You can't import a .pdf here. 

Take a screen shot of the object (not the whole screen!) in Preview. Save as .png (default). Use the picture link in the toolbar to import your graphic. 

Glad you have found a solution, but it is not the obvious one. You can try this next time, it is perhaps more intuitive:

Enter 9 eighth notes in your 2/4 bar. You will need to deselect "Jump to next measure". I use Speedy Entry, so this is simple. 

Select the tuplet tool, click the first note and create a tuplet of 3 quarters in the time of 2 quarters, click OK

Double click the first note (Single click will highlight the existing tuplet, double click to make a new one when there is already one there). Create a tuplet of 3 eighths in the time of 2. Click OK

Click the 4th note and make a 3:2 eighths triplet. Click OK

Click the 7th note and do the same again. 

You will now need to do a bit of manual adjustment, and maybe join/break some beams to make it show exactly as you want.