If it wasn't L5R's most successful ad, it was certainly the one that caused the most discussion. People remembered it, quoted it, and misquoted it, for years after it ran.
The ad was running around the time Crimson & Jade was released (which brought to card pool to about 900). The ad below was scanned from InQuest #28 (cover dated August 1997), but was also running in The Duelist, Scrye, and so on.
Here's what Ryan Dancey, the ad's creator, had to say about it in 2002 on the l5r-ccg list:

The plan was first announced in a small Crimson & Jade advertising
booklet that was distributed in The
Duelist #16 (I think). It
was not a small little thing buried in the text; it was prominently
announced on the back cover, with a "Pro Tour" logo, the whole deal.
Here's what it said:
The announcement was, of course, greeted with panicked protests: "You're going to turn this game into Magic!"
The idea was dropped for various reasons. I bet it turned out to be too big an organizational problem. Ryan Dancey posted one email about it after the plans were scuttled where he described what he had planned.
Basically, every event would be an L5R mini-con, with things like costume contests, LARPs, art show, and lots of L5R related stuff besides just playing the CCG. The finals would have been a big event held at the Imperial Palace Casino in Las Vegas. (Yep, Ryan even had the site for the finals picked!) There was a lot more too it than that, but those are the details I remember.
Interestingly, after Ryan posted just what he had in mind, even some people who had first decried it as a stupid idea grudgingly said, "That... sounds... sort of... cool..."