Search on Google you will get plenty of papers on Brassica transformation.

The Sparkes paper (Nature Protocols (2006) v.1 no.4 p.2019-2025 might provide help; though the protocol is specifically geared for Nicotiana benthamiana, perhaps following the suggestion of growing the plants under lower light intensity might result in a leaf morphology more permissive to successful infiltration of the inoculum into the substomatal chamber. The p19 silencing suppressor is also quite useful, and you might consider using it. There are publications which outline transient assays in Arabidopsis, though Brassica is quite a bit different.

1. You can try using the 'tobacco method (with a needle-less syringe)' on the Brassica leaves to see whether it  works.

2. Some people also dipped the cut small plant leaves in Agrobacterium solution, in a beaker, and place the beaker in a vacuum chamber (see attached picture) to 'force' the Agrobacterium get into the leaves by vacuuming for transient assay.

The assay can be performed as described in'Soybean GmbZIP123 gene enhances lipid content in the seeds of transgenic Arabidopsis plants'. 

  'The 1.2 kb sequences upstream from the ATG codons of SUC1,

SUC5, cwINV1, cwINV3, and cwINV6 were inserted into pGWB435

to generate promoter:LUC reporter constructs using Gateway ®

technology (Invitrogen). The reporter plasmid and the pROK2 plas-

mid containing the 35S:GmbZIP123 construct were transformed into Agrobacterium tumefaciens strain GV3101. The Agrobacterium

strains were incubated in Luria–Bertani medium and finally resus-

pended in infiltration buffer (10 mM MES, 0.2 mM acetosyringone,

10 mM MgCl 2 ) to an ultimate concentration of OD 600 =1.0. Equal

amounts of different combined bacterial suspensions were infil-

trated into the young leaves of the 5-week-old tobacco plants using

a needleless syringe. After infiltration, the plants were grown first in

the dark for 12 h and then with 16h light/8 h dark for 48 h at 24 °C

before charge-coupled device imaging. The leaves were sprayed with

100 µM luciferin (Promega) and placed in the dark for 5 min. LUC

activity was observed with a low-light cooled charge-coupled device

imaging apparatus (iXon; Andor Technology). Experiments were

performed with three independent biological replicates.'

   This transient  expression assay system might be used on Brassica. You can have a try. BTW, The p19 silencing suppressor is required.