Yes 'blaw' can mean 'boast' i.e. 'blow' - as in blow your own horn.                                                            

'Blaw' is used twice in the song,                                    

1.'Ye gaitherers of blaw'.
2. 'Wi' a bag o' blaw upon my back, my face as broon's a toad'.

In neither context does 'boast' make any sense. 'Oatmeal' does.
According to the Scottish National Dictionary 'blaw' is 'tinker's cant for oatmeal'.

What about 'toad'? I've seen it given as meaning 'fox'. But not in the SND.

Any thoughts or ideas on this one?