I just noticed that my rowspan suggestion with ellipsis would take relatively long to be noticed by the parser (when it gets to cell C3) and only then can modify the rowspan attribute of the element C2. I don’t know whether this would slow it down a lot, but i thought that alternatively we’d just need to find a symbol for rowspan which we simply place in the same position as the repeated bars:
Visually i’d propose the exclamation mark (!), because of it’s vertical nature and the arrow-like downwards pointing of the point below it. It’s also relatively symetric to the double bar syntax, (repeat the ! if bigger rowspan is needed). Just like in HTML, the cell below would simple be omitted. Only disavantage I see, is that it’s relatively difficult to spot the difference between the bar and the exclamation mark. (The full-stop would probably be used to often as the last symbol and therefore generate unwanted behavior of the table. I doubt however, that there’s much shouting in tables.)
| A1 | B1 | C1 | D1 |
|------|-----|------|------|
| A2 || C2 !| D2 |
| A3 | B3 | D3 |