I've seen mention of cervilets (with a variety of spellings) being worn under greathelms. But I've been unable to find additional info or pictures.

Anyone have more information they could provide?

think of a bassinet or a skull helm (steel bowl). most effiegies have men with basseinets on while resting their heads on the great helms

The term you're looking for is cervelliere. It's a progenitor of the bascinet and is a simple steel skullcap. These were often worn under the coif, which gives the knight a weird swollen-headed look. Please see here for a little more info.

Be careful with this. I do not think all knights with greathelms wore an under helmet. In some of the equipment lists I have gone through indicates this. On the other hand it is very definate it happened. It is clear in a picture from the Luttrel Psalter circa 1320-50 (depending on who you believe, I tend closer to 1320.) In the 13th century I have found very few references to this so I somewhat think it may be a very limited phase of progress between great helm to bascient. Perhaps like the chains attached from 'plates' or breastplate to the helm, sword, dagger etc and alliettes, another quick step in the world of armour (although debatable if the alliet is really armour).
It is also possible that the greathelm was tossed off in battle and this skull cap was what was worn for view and ventilation. I do not think the helmets the knight's are laying their heads, (great helmet) in their effigies are evidence they were used in war at this time as this is done into the 15th century and after with tournement helmets not war helmets, helmets that had been totally abandoned except tournements in the late 14th and 15th centuries. I assume the helmet under The Black Princes head for example is a tourney helmet and in war he'd have a nice visored bascinet. By 1360's it would seem very likely. It seems likely that was always the case as far as effigy helmets under the knights head as well. It is possible in the Luttrell Psalter it is the knight going to Tourney not war but that is just another possibility. I can scrounge up some pictures if you like but I'd need an email to send it to.

Randall