Here's a list of the top 5 slugging percentage seasons for Brewer minor leaguers over the last 10 years (min 350 PA):

*0.605 - Joe Dillon (Nashville 2007)
0.605 - Lance Burkhart (0.678 in 233 AB for High Desert / 0.506 in 170 AB for Huntsville - 2001)
0.594 - Jim Rushford (0.625 in 259 AB for High Desert / 0.455 in 187 AB for Huntsville - 2001)
**0.569 - Prince Fielder (Nashville 2005)
0.553 - Jeff Deardorff (0.554 in 260 AB for High Desert / 0.552 in 201 AB for Huntsville - 2001)

*Dillon also slugged 0.500 in 76 AB for Milwaukee that season.
**Fielder also slugged 0.458 in 59 AB with Milwaukee that season.

A little more about the leader, Joe Dillon

(photo from the mlb.com)

Drafted:                                   Drafted by the Royals in the 7th Round in 1997 from Texas Tech University
How Acquired:                       Signed by Brewers as a free agent during spring training after being released by the Marlins.
Years with Brewers:              2.  Claimed on waivers by Oakland after the 2008 season.
Best Season:                         2004 - Hit .329/.406/.669 with 46 doubles, 39 HR and 117 RBI between AA and AAA for the Marlins. 
Last Professional Season:  Traded by Oakland to Tampa for Adam Kennedy.  Split 2009 between Tampa Bay and Durham (AAA).  Hit .244/.340/.333 in AAA. 
Other Notes of Interset:        Dillon has spent time in the following organizations:  Kansas City, Minnesota, Florida, Yomuri Giants, Milwaukee, Oakland, and Tampa Bay.

Joe is the single season HR leader at Texas Tech with 33 in 1997.

Dillon started 21 games for the Brewers over two season at 5 different positions. (1B, 2B, 3B, LF and RF)

Dillon retired and did not play due to back problems in 2003 and 2004.  During that time he coached for Texas Tech University.

2009 Leaders (min 350 PA)

0.506 - Adam Heather (0.501 Nashville / 0.550 Huntsville)
0.499 - Brendan Katin (Nashville)
0.459 - Caleb Gindl (Brevard County)