Thanks for your question. 1rst half betting in baseball isn't nearly as popular as is halftime betting for football or basketball. (You are the first customer I can remember who has asked about it!) However, if enough members request it, we will certainly look into adding it as a new feature.
I think I may have requested the same thing years ago
The fact that the 1st 5 lines in MLB are not as popular as 1H betting in other sports might lend itself to some softer lines. There might be some high percentage SS's that only apply to the the 1st 5.
Having this info might be a nice niche for The Sheets