I just want to add an idea just by using templates, but we need multiple checkbox fields with single value. Then modify the templates so it looks like one input.
For example, lets say we have 4 values, A, B, C and D
Option A:
Key: oa
Hide Thread Field: check all.
Input Field Type: Checkboxes
Value List: A
Multiple Value Delimiter: space or whatever
Filtering Mode: Exact match
Option B:
Key: ob
Hide Thread Field: check all.
Input Field Type: Checkboxes
Value List: B
Multiple Value Delimiter: space or whatever
Filtering Mode: Exact match
Option C:
Key: oc
Hide Thread Field: check all.
Input Field Type: Checkboxes
Value List: C
Multiple Value Delimiter: space or whatever
Filtering Mode: Exact match
Option D:
Key: od
Hide Thread Field: check all.
Input Field Type: Checkboxes
Value List: D
Custom Modify Error:
This one forces user to select at least one options.
Multiple Value Delimiter: space or whatever
Filtering Mode: Exact match
Insert this code inside the form (for example before {$posticons}) in template_prefix_newthread and template_prefix_editpost_first
Then for filtering interface, insert this code inside the form in template_prefix_forumdisplay_searchforum_inline (don't forget to activate the Enable XThreads' Inline Forum Search setting: