Without any images its difficult to judge what you're trying to achieve, but -usually- the answer to any dynamic box-drawing is frills processing. You need to flag (</fg>) the starting line and the ending line, then use frills processing to find those flagged lines, which gives you the starting y-origin and the ending y-value, do the math to give you the depth. The width is probably related to the main block width. 

All of these are able to be determined from !variables. (XyMacro manual)

Yes Mark is correct. You will need frills processing. But I would like to add to Mark's answer that you there is no need to flag the first and last line with the fg macro. In frills processing you can simply open up the first line in the first block and the last line in the last block of a page and read out the necessary information (yes with !variables) to draw your box

BUT
you might also have a look at the support XPP gives you for the border CSS property. This allows you to draw a split box over all pages by a simple CSS rule (e.g.  border: medium solid red;). But you need to set up your styles in CSS (and not IF).
 

Yes Mark is correct. You will need frills processing. But I would like to add to Mark's answer that you there is no need to flag the first and last line with the fg macro. In frills processing you can simply open up the first line in the first block and the last line in the last block of a page and read out the necessary information (yes with !variables) to draw your box

BUT
you might also have a look at the support XPP gives you for the border CSS property. This allows you to draw a split box over all pages by a simple CSS rule (e.g.  border: medium solid red;). But you need to set up your styles in CSS (and not IF).
 

Thanks Bart and Mark for the suggestion. I was able to draw the box by using system variables within main blocks itself.