Nah, the Coppermine and Mendicino cores did have different pin assignments, THG even had an article on how to mod your old Celeron Slotket to support newer Coppermine cores.
At any rate, incompatible CPU's will black screen by default, the only way to know if they're fried is to try them in a compatible system.
I have followed tutorial on modding a celeron only board which used calerons from 233MHz up to 1.1GHz (both mendocinos and coppermines) to use a P3. The P3 had some differences in pin arrangement but the res was only voltage adjusting. That board officially supports coppermine P3s and slightly after them, coppermine celerons were released, so there's no reason why it should have blacked out. I have tried that celeron before without modding, on 2.0V, and that could have fried it.
I guess the modding article you mention is pretty much about Vcore adjustment, because an OCed coppermine run at most on 1.80-1.85v and mendocinos used 2.0V.
if the board (unmoded, default) supports mendocino/covington (250nm) cpus and not coppermine (180nm) cpus, the system will not post, and if i remember correctly will not even power the cpu - feel the cpu die, it will remain cold (all my boards did that when i had em).
If i remember correctly, Intel even conciders 2.0v vcore usage safe for there 180nm cpus, THG even commented saying to use with caution and decent cooling, i must find the article.
Modding deschutes/katmai/covington/mendocino (250nm, 2.0v) boards (well, even 350nm 2.8v clamath cpus aka P2, ~40w @ 333mhz! more then a Tualatin 1400!) to support coppermine (180nm, ~1.75v) cpus is easy as - cut one pin on the cpu and soldier a wire across the back of the socket and presto it works.
Theres more boards that support the mod then not, mind you one or two will not work with it, one example is the ASUS P2B, atleast for me it didnt work at all.
My favourites in there day - ABit BH6 (no FSB133 support
🙁 ) and the AOpen AX6BC - both worked a treat, both Slot1.
The greatest product MSI ever made - the MS6905 slocket adapter, mind you they fcuked it up so many times and made what 4 revisions to support coppermine finally and even tualatin cores? i still have one or two!