I've been trying to figure out how to do a BX runecaster for a while. On the one hand, I want to keep the concept in line with a shaper who inscribes items with runes. On the other, I know that to be viable as a PC, the class must be functional enough to bring the character's abilities to the table on a daily basis. After all, what good is a runecaster if he serves no function in the day-to-day actions of the party (e.g., dungeon crawling). I think that's one of the reasons the runecaster has oft been relegated to the role of NPC—somebody the PCs visit to request a special item, then come back to in 1d12 days to retrieve their item.
I think I've found a fairly elegant answer that solves my issues. I will say, I don't think I would have come up with this answer had I not first developed my psionics rules. It was the psionics rules that made me really think about mechanics that are "simple enough" for BX, but still retain the spirit of the underlying subject. So here are the basic concepts...
I'm essentially converting the standard "required shaping time" (which, in the past, has been a random die roll to determine the time taken, e.g. 1d6 days), and assigning the rune a "Shaping Value" (essentially a complexity rating for the individual rune). Then, the number of runes that can be shaped per hour (or per day) is determined by the level of the runecaster (see the chart below). So while adventuring, a runecaster can shape 1 hour's worth of runes (after a full 8 hours rest). While not adventuring, the runecaster can shape 8 times that amount (assuming the full day is dedicated to shaping runes).
The runecaster may only have a maximum number of active runes equal to his or her level (e.g., a 7th level runecaster could have 7 runes active at one time). The number of active runes is unrelated to the shaping value (which only determines how many runes can be shaped in one hour, based on the shaping value of the rune).
Unlike spells, the "level" of the rune doesn't reflect a power level (that's reflected by the shaping value). Instead, it simply denotes how may runes must be known as a prerequisite to that rune (e.g., a 2nd level rune requires knowledge of a particular 1st level rune, and a 3rd level rune requires knowledge of a particular 2nd level rune; I have yet to create a 4th level rune, and doubt that I will).
So that's basically it. The info below should help tie it all together. (I've also worked out the prime reqs, armor/weapon allowanced, etc., but that's generally unimportant right now. Though it should be noted, I'm really looking at upping the XP requirements for the dwarven runecaster.)