Where can I find the maximum time-in-grade for Officers, such as for below-CPT Officers with no slots to promote into?

Since you talk in your comment about RC promotions in particular, you'll probably want to look at AR135-155, which has a table specifically listing minimum & maximum TIGs and other requirements (Tables 2-1, 2-2, & 2-3). That covers the initial question, but the comment also mentioned auto-promotion without a vacancy. You'll want to check with your branch personnel at HRC to be 100% sure, but my understanding was that, once selected by a board, if you didn't find a vacancy, you'd be auto-promoted on the day prior to the next anniversary of your previous date of rank, and would then be given 6 additional months to find a vacancy. If still unable, you get involuntarily transferred to the IRR. (If you are already IRR, then it's always the day before your next DoR anniversary.) I'm sorry that I don't have a reg to point to for that part, hence the suggestion to confirm with HRC.

I found references in 10 USC that qualified O-2 Officers would be promoted into O-3 regardless of availability of vacancy, but I cannot determine if that also applies to Reserve Component (RC) Officers, and I have been unable so far to find a similar regulation for RC Officers.

I'm not sure that I understand the no slot thing. While command slots can be tight if you aren't in an Infantry, Armor or Engineer Unit, there are generally lots of staff slots where they can park you as an O3 until a command slot opens up. Have you talked to personnel people at your state headquarters yet? An S1-S4 are not Branch specific.