Setting "\tempdir_path" before first start

[Jakob Jakobson]

Dear developers,

first of all: Thanks for the great work that you have been put into LyX!
I was trying to get LyX packaged for flatpak ( see also 
https://www.lyx.org/trac/ticket/11143 ) when I ran into a serious issue: 
I need to set the "\tempdir_path" before the first start of the program. 
Is this possible and how can it get done?
The background is that flatpak wants to sandbox applications as much as 
possible and any access to the system directories has to be explicitly 
granted. This leaves us with to possibilities:
a) As the access to the home directory is useful anyway, we set the 
"\tempdir_path" to a directory within the home directory. However, this 
should be done before the first start of LyX for a smooth user 
experience.
b) Any system directory can be accessed if the access is explicitly 
granted. However, granting access to "\tmp" does not do the trick as 
this grants access to "\tmp" and NOT its subdirectories (as far as I 
understood). With LyX saving its temp-files in sub-directories of "\tmp" 
and not in "\tmp" itself, I don´t see how this would get me any further.

My proposal therefore would be to go with option (a) but I don´t know 
how to do that. Do you have any ideas?

Kind regards,
Jakob