trying last two suggestions offered....

in aliases.ini:
/temp_ignore ignore $+(-u, $1) $2 8 $chan

then, invoked by:
/temp_ignore 180 test_nick

mIRC reported:
* Added test_nick!*test@*.dsl.lsan03.pachebel.net to ignore list for 180 secs

but... using the address book editor to examine the ignore list showed the following:
test_nick!*test@*.dsl.lsan03.pachebel.net (pcntdik) - #test_channel
...and when I selected that entry for editing, the "Remove in ___ mins" box was unchecked

also, mIRC did NOT time-out the ignore entry and remove it

I was curious if the remove mechanism works at all, so I manually edited the ignore list entry for a 3 minute timeout and waited....

checking at roughly one minute intervals, I could see that mIRC was decrementing the time remaining towards zero... eventually, mIRC timed-out the entry and removed it from the list

then, I altered the alias to be:
/temp_ignore ignore $+($chr(45), u, $1) $2 8 $chan

and invoked it again, after first verifying that the prior entry in the ignore list had been removed

same results as before, unfortunately

my conclusion: there are three differences between the expected behavior (from reading the help file) and actual behavior:

(1) the -u option, despite a message from mIRC indicating that the requested action has been taken, is completely ignored in practice

(2) the -u option specifies a time in seconds, but the address book editor specifies time in minutes

(3) manually editing the ignore entry to specify "Remove in ___ mins" does work -- I just can't figure out how to do this within an alias