What you really want isn't necessarily to insert ">" in the flowed text at the beginning of every line. What you really want is for every line in a given block to have a ">" at the left of it.
You could potentially do the latter by:
1) Creating an image of the ">" character, with its height from top to bottom equivalent to the CSS
line-height value you plan on using. Apply that image as a background on the container of the text you want this "inserted" into. Have it repeat vertically, but not horizontally. Pad the container the width of the character/image.
line-height property for the container you're working with. Then, assuming you're not artificially setting the height of this container, the number of "lines" of text in the container is its
offsetHeight divided by the computed line-height. At this point, create a separate container that has nothing but that number of >'s as its content. Position it to the left edge of your content container.