May an employer operating two qualified separate lines of business (QSLOBs) offer a 401(k) plan to employees in one QSLOB while offering no 401(k) plan (or other qualified plan) to the other QSLOB? If yes, does the answer change if the employer adds additional QSLOBs and offers separate 401(k) plans to each QSLOB except for one?

If your SLOBs are QSLOBS you don't have to aggregate them for coverage or testing. The SLOB that has benefits must pass coverage separate from the SLOB that may have no benefits and SLOBs that do have benefits. In essense you treating each SLOB as separate employer and ignoring the others in the group.

No the answer doesn't change. Just watch out for the substantial service employee if you start adding SLOBs.

What about the requirement to pass the nondiscriminatory classification "gateway" test (410(b)(5)) on an employer-wide basis? Do you agree that this test applies on an employer-wide basis, treating employees of the nonparticipating QSLOBs as not covered by the plan?

Yes you have to pass the gateway too. Depending on the size of the your non covered group it could be issue.