I have found rule 94 (iv) which states that any horse rated below 46 entered in a Flat Race handicap shall be allocated a weight which corresponds with a handicap rating of 45. So, If you are entered in a 46-60 handicap and the top rated horse is only 57, it effectively becomes a 45-57, not a 43-57.
The penalty is then added to your allocated weight.
The purpose of publishing ratings below 45 is for elimination sequences in oversubscribed 0-45s, sellers, claimers and maidens. Plus, of course, if you have an oversubscribed 46-50 handicap, the horse rated 44 and the horse rated 32 will obviously both compete off 46 (or 45 if the topweight is below 50, as above), but the horse rated 44 has more chance of securing a run.