This leads me to mention a theory I have about the Invisible Stalker spell. The six demon varieties we all know so well were added in Eldritch Wizardry; before that, the closest thing resembling a demon is the invisible stalker, at 8 hit dice, is about the same level of menace as a Type 6 demon. The stalker's powers are not as detailed as those of the EW demons, but there's a suggestion that stalkers are more than just invisible fighters with flight and faultless tracking ability.
For example: An Invisible Stalker is ordered to: "Guard me against all attack, and see that I come to no harm." In order to faithfully fulfill this endless duty the Invisible Stalker will have to take the Magic-User to its non-dimensional plane and place him in suspended animation, and assume this is accomplished whenever a 12 is rolled with two six-sided dice, checking either daily or weekly as the campaign progresses.My thought is that the invisible stalker is the original version of "summoned demon", and the 6th level Invisible Stalker spell is meant to be the OD&D equivalent of the 7th level Cacodemon spell in AD&D. This still doesn't quite meet JB's needs, since wizards have to be 12th level to use 6th level spells. But there's always scrolls, and the Invisible Stalker spell doesn't require as much paraphernalia or extended ritual as Cacodemon, making scrolls of summoning not too out of place. (Of course, if all he wants is an old 1st level guy who can summon demons at great risk, I would suggest checking out the Conjurer, or maybe the Sorcerer for a slightly broader but more erratic approach.)
The suggested mechanic in the Invisible Stalker quote above might be some kind of reaction roll, but it could be re-purposed as an open-ended powers roll. Assume that stalkers can perform any action with results equivalent to a 6th level spell or below. Double the spell level to get the target number; rolling that number or higher means the stalker succeeds. Fighting, tracking, or retrieving doesn't require a roll, because it's part of the stalker's bailiwick.