Within the constraints of the file system, changes made to the name of an item in DEVONthink are reflected in the file name in the file system (i.e., shown in Finder).
if instead of OCR to Searchable PDF you use the following script (embedded), I think what you are asking for should work (this is assuming I have understood you correctly); please test before using in on important files:
on performSmartRule(theRecords)
tell application id "DNtp"
try
show progress indicator "OCR" cancel button 1 steps count of theRecords
repeat with theRecord in theRecords
if cancelled progress then error number -128
step progress indicator (name of theRecord) as string
set theFile to path of theRecord
set newName to name of theRecord
set newName to newName & "_ocr"
set newFile to ocr file theFile to parent 1 of theRecord type PDF document
set name of newFile to newName
end repeat
on error error_message number error_number
hide progress indicator
if the error_number is not -128 then display alert "DEVONthink" message error_message as warning
end try
hide progress indicator
end tell
end performSmartRule
Edit: this script uses a progress indicator. Whilst OCR opens its own progress indicator, canceling that will only cancel the current file. If the rule/script is run on a large number of (large) files, there would be no option to cancel all if the script didn’t integrate a second progress indicator. A trap for the unwary, so to speak.
The resolution can be set from 150 to 300 dpi. If for your test you used a file which already had a text layer or had previously been OCRd, the results may have been misleading. You might want to add a condition to your smart rule which says Word count is 0; that way your rule only acts on PDFs without a text layer.