"When would you like a viewing?"
"How about tomorrow? Say tenish?"
Does "tenish" specifically refer to ten o'clock am / pm or both?
Can other numbers be used in the same way when talking about time?
You can add the suffix -ish to any number (i.e. time and height and length; any type of measurement). It means, at around / at about.
See you at tenish!
(See you at around or at about 10 o'clock. The speakers would know if it's morning or evening. As is, the sentence you provided doesn't make note of whether they are to meet in the morning or in the evening.)