It is possible to make a limiter that removes the spikes. Thinking on my feet may be back to back (series) diodes. That conduct at say 6W or so. In normal use they’d be high impedance. I’m sure this issue must have been solved by transverter users using other ALC limited QRO rigs.
You’d gain bonus points from me for using an old school 290 with a very modern transverter. 