[MapProxy] Provide Google-style tiles using a non global spherical mercator tile source

[Josh Doe]

I'm trying to provide Google/OSM style tiles from an ArcGIS REST tile
server, however the server uses a unique SRS,
Lambert_Conformal_Conic_Virginia. I was able to find the proj4 string
(SR-ORG:6639), and added it to the epsg file, but now I'm trying to
figure out how to provide the Google style tiles. It would seem I'd
need to setup a EPSG:900913 cache, however from my reading it seems
like for tile sources the cache grid and the source grid must match.
Now I guess I could create a second MapProxy instance to access the
first via WMS to create this cache, but it sounds like I must be
missing something.

My config file isn't long, so I thought I'd reproduce it below for posterity.

Thanks,
-Josh

services:
  demo:
  tms:
  wms:
    srs: ['EPSG:4326']
    image_formats: ['image/jpeg']

layers:
  vbmp_2002:
    title: 'VBMP 2002 Orthoimagery'
    sources: [vbmp_2002_cache]

caches:
  vbmp_2002_cache:
    grids: [vbmp_grid]
    sources: [vbmp_2002_rest_tile]

sources:
  vbmp_2002_rest_tile:
    type: tile
    url: 'http://gismaps.virginia.gov/arcgis2/rest/services/VBMP2002/MapServer/tile/%(z)s/%(y)s/%(x)s'
    grid: vbmp_grid
    origin: nw
    coverage:
      bbox: [-384269.977874641, -48883.4062351487, 404548.778455541,
476583.633639972]
      bbox_srs: 'SR-ORG:6639'

grids:
  vbmp_grid:
    srs: 'SR-ORG:6639'
    tile_size: [512, 512]
    res: [1058.33545000423, 423.334180001693, 211.667090000847,
105.833545000423, 52.9167725002117, 26.4583862501058,
12.7000254000508, 6.3500127000254, 3.1750063500127, 2.11667090000847,
1.27000254000508, 0.63500127000254, 0.31750063500127]
    bbox: [-37232800, -477758.7842, 697942.5282, 45581000]