Is such a thing feasible without messing with some crucial bit of math which I haven’t yet noticed?
For those of you unfamiliar with S&W, it has only a single save, which is modified by circumstance (for instance Magic-Users are better at saving against spell effects) and sometimes by a relevant ability score. You can get a more thorough description at Swords and Wizardry’s SRD
The S&W ‘single save’ progression doesn’t compare very cleanly with their ‘save table’ progression - the single save lowers much faster.
I don’t know that I’d use S&W’s progression as-is for ACKS, as it is much better in most circumstances.
What I’ve done in ACKS for playing with my kids is to average together the saves per progression type, and calling that the ‘single save’ progression for that class type. That keeps the general progression intact, and doesn’t greatly interfere with the ‘math’ of the throws.
Math-wise, you’re either losing or gaining from 5% → 15% from any given category - for example, the Cleric has a very good Poison/Death save. On a straight averaging, she’s losing ~15% chance to save vs that (because of her high Blast and Spell saves, which she gains 10%->15% on due to the averaging) , which could easily be fixed with a general bonus to that save type if you want the Cleric to be strongly-typed for that type of save.
Looking at the ACKS tables, the Cleric saves well vs. Poison/Death, and the Mage vs. Staves/Wands, and the Thief and Fighter don’t stand out much anywhere.
In fact, from an yet unpublished blog post, tables up to level 6:
Fighter
Level PP PD BB SW Sp Avg
1 15 14 16 16 17 16
2 14 13 15 15 16 15
3 14 13 15 15 16 15
4 13 12 14 14 15 14
5 12 11 13 13 14 13
6 12 11 13 13 14 13
Mage
Level PP PD BB SW Sp Avg
1 13 13 15 11 12 13
2 13 13 15 11 12 13
3 13 13 15 11 12 13
4 12 12 14 10 11 12
5 12 12 14 10 11 12
6 12 12 14 10 11 12
Cleric
Level PP PD BB SW Sp Avg
1 13 10 16 13 15 13
2 13 10 16 13 15 13
3 12 9 15 12 14 12
4 12 9 15 12 14 12
5 11 8 14 11 13 11
6 11 8 14 11 13 11
Thief
Level PP PD BB SW Sp Avg
1 13 13 16 14 15 14
2 13 13 16 14 15 14
3 12 12 15 13 14 13
4 12 12 15 13 14 13
5 11 11 14 12 13 12
6 11 11 14 12 13 12
There’s a bit of tradeoff here. It’s hard to say whether losing 15% on Poison/Death is balanced by gaining 15% on Blast/Breath and 10% on Spells - it’s situational, and often decided by a random monster table.
(and not as big of a deal for me as I’m not yet ‘playing for keeps’ with my girls)
That could be offset partially by ability score bonuses as well - if you figure a Fighter may have a better chance to have a good STR or CON score, let him use that +1/+2 bonus to offset saves that would have been vs. Poison/Death or Petri./Para. - that steals back the points lost to averaging.