BSQ is band sequential. The GDAL doc is here
Extract from ESRI whitepaper: Band sequential format stores information for the image one band at a time. In other
words, data for all pixels for band one is stored first, then data for all pixels for band two,
and so on. in http://downloads.esri.com/support/whitepapers/other_/eximgav.pdf on page 8.
If there's only one band there is no difference between BIL and BSQ.
If the NBITS isn't specified then it can be calculated by the
size of file (in bytes) / (NROWS * NCOLS). This should give you either 8, 16 or 32. If it's 32 it may be float or 32 bit integer (long), you will need this in bytes 1, 2 or 4 respectively.
The hook point is usually upper left, so you find the difference from your coordinate to the reference point (in cells):
Row = int( (ULYMAP - Your.Y) / YDIM )
Col = int( (Your.X - ULXMAP) / XDIM )
That is assuming your point is in the same spatial reference as the image, if not project the point - or if all of your requests are going to be in the same spatial reference that's different to the raster consider projecting the raster.
With the row and column known calculating the location in file is
(Col * NCOLS * bytesPerCell) + Row * bytesPerCell