I also happen to be going to Scotland in April, so if this happens to be the end for available online research, do you happen to know where I can go to find this information? I just think it's crazy that it's so difficult to find any information on an ancestor from the late 18th century to mid 19th century in Scotland.
Basically, if it's not on Scotland's People the information probably does not exist. The most common reason is that for whatever reason (parental negligence, careless clerk, forgetful minister) the baptism either was not recorded, or the record has not survived.
Yes, there is a
very small number of registers that for one reason or another have not, or have not yet, made it to Scotland's People.
A handful of these, held in the National Records of Scotland, cannot be digitised, either because the congregation that owns it has not given permission, or because the owner cannot be found.
The rest of the 'missing' ones are mainly the registers of the Episcopal Church, which are all over the place, some in the churches, some in diocesan archives, and some in university or local archives. For instance the University of Dundee has, I believe, some Episcopalian baptism registers.
There might also be the odd register from another denomination, such as Congregational, Methodist, Baptist, Society of Friends lurking somewhere but the chances of finding any such are small. I am certainly unaware of any repository that holds any of these.
Do you not think that, if the records were available, one of the helpful people on RootsChat would have told you about them already?