Event quests have a specific signature, which are different between XX and GU. The XX crashes because the associated exception is not properly handled. When GU was released to NX they added a handler and so the game do not crash but instead show a clear error message.
You are asking for a workaround. I can only tell you what I've done. I own a CFW 3DS with XX installed, and so a friend of mine but with a very old save file. His save file contains the five Gudetama/Nisetama event quests that are not longer available for download, and I'm a completist so I wanted those quests very badly. What I did? I followed
this youtube tutorial about using Citra emu to move my saved character to my friend's save file which contains all removed DLCs. A few months later I also learned that you can do it manually by following the method described
here. The "clear" state of event quests do not change if you replace the DLC data because it only contains the quests data and the "clear" state of quests is character-specific, so your DLC progress is safe.
Why I'm telling you all this? Because now I have the theory that you can actually add all GU event quests from a GU save file to your XX save file, or at least replace the old XX events with the ones available in GU, by following the manual method above. Doing so will allow you to take on GU event quests without crashing, however you may still crash XX users that don't have such quests, but they're a lot less common so I think it worths. You only need a GU save file (which you can obtain by googling) and the ability to follow basic instructions from the post linked above, to eighter copy the DLC data to your save file or to convert the GU save to a XX save (by using
this tool) and put your character on it. I will do that as soon as I have the time (I'm very busy right now) and I will update this post with some feedback. Hope this post can be helpful.
Kind of off-topic but for completion purposes I'll add bellow the five event quests that are not longer available for download and I'm very, very happy for finally having them. The first three are from MHX, and the other 2 were added in MHXX. Some of them gives you a very nice gear for your hunter (try rainbow or skin-color on it) and your palico as well. Please note that
you shouldn't use them online or you will crash GU players since those gears were dummied out in their games. XX-exclusive weapons don't crash GU players often, but gear does.
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