Can you redesign the pattern block (or close) that can then be repeated as with any other standard background-image: ? You may have some reverse engineering options.? 100kb for a background-image: - ouch.
I built a site for a graphic-design/printing company once. The owner turned the decisions over to his chief graphic designer. OMG. It wasn't so much that they knew nothing about website design. The big problem was that they were expert print people and absolutely could not bring themselves to understand that print and internet are apples and oranges. She designed the background-image: that they wanted and it was monumental. It wasn't even that good and could have been redesigned into several graphics that incorporated the key graphic elements throughout the page at a very reasonable size. She wouldn't have it. It was to be used 'as is', only file size optimization allowed (which was pretty pointless. The .eps was like 20GB. She just went to town with layer upon layer of graphic doo-dads:)) Had to get the okay just to resize the darn thing to a size that more than 1% of their users would see.
It was their dime, but I made sure that there was plenty of documentation regarding my position. Once they signed off on the final product I fired them. I didn't tell them until they came back to get some updates, a special form page for users looking for spec requests, and such. I just told them that my workstack was closed for several months.
Eventually, they scrapped the site completely and put up a whole new terrible site. I am sure it was that same 'chief designer' that wanted what she wanted and didn't want to hear that there HAD to be some give to the take.
I know just enough about print work to ask lots of questions about 'best practice' when I am involved in a print project. I've learned a fair bit I think, but don't pretend to in any way be a print person. I enjoy learning what I can where I can but don't pretend to be right person for the job if I am not.