I don't think the suggestions made in that thread are going to help in 
terms of org-mode recognizing a line as being a list item.
What I tried that I did think might work was to customize 
org-list-full-item-re:

org-list-full-item-re is a variable defined in ‘org-list.el’.
Its value is
"^[     ]*\\(\\*(?:[-+*]\\|\\(?:[0-9]+\\|[A-Za-z]\\)[.)]\\)*\\(?:[     
]+\\|$\\)\\)\\(?:\\[@\\(?:start:\\)?\\([0-9]+\\|[A-Za-z]\\)\\][     
]*\\)?\\(?:\\(\\[[ X-]\\]\\)\\(?:[ ]+\\|$\\)\\)?\\(?:\\(.*\\)[     
]+::\\(?:[     ]+\\|$\\)\\)?"

   This variable may be risky if used as a file-local variable.

Documentation:
Matches a list item and puts everything into groups:
*group 1: bullet*
group 2: counter
group 3: checkbox
group 4: description tag

to add '!' and '?' to '-+' in the group 1 expression, but that didn't do 
it. ( *[-+*] => [-+!?*]* )

On 10/24/2018 5:47 PM, Grant Rettke wrote:
> On Sat, Oct 13, 2018 at 1:56 AM Colin Baxter <m43cap@yandex.com> wrote:
>>>>>>> Tom Burbage <tiburbage@gmail.com> writes:
>>      > When I use simple lists, I would like to be able to mix in '!' and
>>      > '?'  with '-' as these are sometimes more expressive of what the
>>      > list item represents.  My request is that the list of characters
>>      > org-list recognizes as being a valid "bullet" be customizable by
>>      > the user.
>>
>> Have a look at https://emacs.stackexchange.com/questions/35626/how-to-make-my-own-org-mode-text-emphasis-work-again.
> Which code are you recommending?