A few more thoughts:
I've come to realize that at least for home schooling or other one-on-one instruction, pretests may be largely unnecessary. When you're familiar with a child's abilities and can see that working a particular type of problem is very quick and easy, the child complains of boredom or expresses a wish to get on to the next section, etc. you can skip some repetitive material on an ad-hoc basis. The material itself fulfills the function of a pretest then, and the structure of Singapore Math lends itself well to this. Pretests are obviously more useful for compacting in a classroom setting, since a teacher may need a number of questions right and wrong to be able to check off on a progress checklist.
With Singapore math, I think it would be possible to quickly construct one's own pretests. This sort of pretest would also be palatable to a (reasonable) teacher, if the Singapore Math is planned to be administered at school as part of differentiation. And if the teacher preferred to do it, it wouldn't take long at all, and could be reused for future children also taking Singapore Math.
For constructing a pretest for a Singapore Math section, one could skip to the review pages for that section, and pick one or two representative questions of each type, either from the review pages or the workbook. I guess retyping or otherwise copying the questions (and answers) would be in order, to make it easy to reuse the pretest. Or one could simply give all of the review questions as a pretest, which might involve some repetition but would still compact quite a bit. If a child failed the review questions by getting a lot of them wrong, it might actually help reinforce and strengthen for them to do the same questions again later after learning the concepts.
Another way would be just to skip to the review section and have the child do the word problems there. This way would let the child skip almost all the material in the section if they displayed mastery, while still giving them extra problem-solving practice, and hopefully avoid any boredom. The only problem would be if there were no word problems in the review section for a concept taught in the section, so one would have to be alert to that possibility.
One general concern with compacting math this way might also be that a child might need a little more practice on certain operations to improve speed and/or accuracy. Good judgement in how many problems to include in a pretest would address that concern. The Singapore Math review problems do include more quickie operation-practice types of questions than word problems, so that might be a good reason to just use the entire review part as a pretest.
I think that with a teacher receptive to the idea of curriculum compacting, volunteering to create pretests could help.
