The original manuscript is not extant here, but between the extant fragments there is definitely room for the subordinate conjunction that after except. It is quite possible that the that was deleted in the original manuscript, as suggested in the transcription of 𝓞 in volume 1 of the critical text:
( e) a sign & now it came to pass that Alma said unto him EXCEPT < > YE SHALL SHEW M
There is no that in the printer’s manuscript.
Elsewhere in the text there are 124 occurrences of except followed by a clause. In only one case in the earliest (and the current) text do we have the subordinate conjunction that following except:
But here the that may be more connected with the preceding verb say than with except; that is, the sentence could be thought of as involving ellipsis: “we shall say no more concerning their preaching except [we shall say] that they preached the word and the truth”.
The usage except that followed by a clause was more common in earlier English, as in the following example from the Oxford English Dictionary under except (spelling regularized):
There is one example in the King James Bible: “and except that the Lord had shortened those days / no flesh should be saved” (Mark 13:20). Otherwise, the Book of Mormon and the King James Bible have no examples of except that. On the other hand, the original Book of Mormon text had numerous examples of the archaic conjunctive forms before that, after that, and because that (which are also found in the King James text).
We should also keep in mind that some other word besides that could be the reason the lacuna in 𝓞 is longer than what the text in 𝓟 requires. For instance, the previous line ends with the verb deny, so perhaps the next line began with a direct object for deny, as suggested by the note for this line of 𝓞 in volume 1: these or them before except may have been deleted or lost when copying from 𝓞 into 𝓟. Thus the best solution is to assume that some word (such as that after except or perhaps these or them before except) was written in the original manuscript and then deleted. In other words, we should follow the earliest extant reading, the reading in 𝓟, for Alma 30:45: “except ye shall shew me a sign”.
Summary: Accept for Alma 30:45 the reading in 𝓟: “except ye shall shew me a sign”; although there is room in the lacuna of 𝓞 for except that or some other textual expansion, we cannot be sure what the actual reading was there, so we will follow the earliest extant source, 𝓟.