Do you mean the asteroid belt?
Here are my entries for the system and the solararch.ini.

[object]
nickname = EA11_field_1
pos = 0, 0, 0
visit = 1
archetype = planet_fake_asteroid

[Solar]
nickname = planet_fake_asteroid
ids_name = 60203
ids_info = 60204
type = PLANET
mass = 1000000
solar_radius = 16500
LODRanges = 0, 99999990
shape_name = Asteroid_shape

As Moonhead wrote, the Asteroid_shape above is a custom made Spaceobject like the icon for Jupiter. This was the source image, I just removed all the unnecessary stuff and added transparency.
If you want to fly trough your system, you could merge the shape with an existing one so that there’s no nameless floating object in the systems center. The downside, as you can see at the end of the video, the Belt disapears when zooming in and as soon as you give it a name, objects inside its solar_radius become unselectable.

I’ve attached the .3db file, just put it in the NEWNAVMAP/SPACEOBJECTS folder and replace the texture with whatever you like.

To tired to type atm.
Hope it helps.

Edit:
Did a bit of research.
For example Luxury liner Shetland br_06_01.
There are 4 people offering bribes. The bartender, 1 policeman br_p_grp an 2 bountyhunters gd_bh_grp.
The bartender offers 3 bribes and 5 rumors. He’s always there so the chance offering a specific bribe or rumor is 12.5%.
The Bretonian Police has a faction weight of 6%. The policeman is offering 4 bribes and 3 rumors.
6%/(4+3)=0.85714%
One of the Bountyhunters offers 4 bribes and 6 rumors, the other one 3 bribes and the same rumors. They have a faction weight of 12% for two people means 6% for one.
So for the first bountyhunter 6%/(4+6)=0.6%
and the second 6%/(3+6)=0.66667%

Looking at mBases.ini, the bartender, the policeman and the first bountyhunter offer bribes for the Bretonian Police br_p_grp 12.5%+0.85714%+0.6%=13.95714 or 13.96 in FLStat.

For Spa and Cruises co_os and for the Bountyhunters gd_bh everyone offers a bribe 12.5%+0.85714%+0.6%+0.66667%=14,62381% or 14,62% in FLStat

For Planetform Inc., the only one not offering a bribe is the bartender so 0.85714%+0.6%+0.66667%=2,12384% or 2.12% in FLStat

Dan’s Calculation for 1 NPC offering a specific bribe would be:

Faction weight / number of NPC of that faction / (number of bribes offerd by NPC + number of Rumors offerd by NPC)

Writing wit a Smartphone is crap!!!

Window Mouse Capture
Works also for other games

I like it! A mix between I-War 2, the X series and Freelancer.

@Bas Here is a Demo from the official site with a 1 hour test period.

Made this a while ago.

http://www.youtube.com/watch?v=efJsP1WBNyI&feature=channel_video_title

The Sol and Jupiter systems are just fake systems for the nav map and the objects act like jumpgates.
The third system is the only one you can visit.