Subsetting before Caching

Question:
I am wondering if it is possible to subset a dataset (via .sel method) before the data is cached.

Reasoning:
My use case is - I would like to cache all the landsat8 data from the s3 repository for a small research (~10km * 10km) station. Currently my catalog looks like (note this subsets the landsat8 tiffs to 60 total - but eventually I would want to use the entire timeseries):

```
plugins:
source:
- module: intake_xarray
sources:
landsat8:
description: Geotiff image of Landsat8 - TESTING.
driver: rasterio
cache:
- argkey: urlpath
regex: 'landsat-pds/c1/L8/033/031/'
type: file
args:
urlpath: 's3://landsat-pds/c1/L8/033/031/LC08_L1TP_033031_{collection_date:%Y%m%d}_20170310_01_T1/LC08_L1TP_033031_{collection_date:%Y%m%d}_20170310_01_T1_B{band:d}.TIF'
chunks:
band: 1
x: 1000
y: 1000
concat_dim: band
storage_options: {'anon': True}
```
Being able to subset before caching would reduce the amount of storage significantly (see example below for my 60 tiff subset):
```
c = intake.open_catalog('test.yml')
l8 = c.landsat8
l8_r = l8.read_chunked()
l8_r.data.nbytes*1e-9
133.64736048

l8_r_sm = l8_r.sel(x=slice(minx,maxx),y=slice(miny,maxy))
l8_r_sm.nbytes*1e-9
0.19845552000000002
```
Is there currently a way of doing this? If not, how difficult would it be to add this functionality? Is it adding an extra argument to the intake-xarray driver (which would look like: _slice: {x:(xmin,xmax),y:(ymin,ymax)}_ in the catalog) or would it need to include modifications to the caching mechanisms deeper in intake?

Thanks!