relative referencing for filenames &etc

BACKGROUND:
I use SAS on a daily basis and one of its most powerful features in a
production environment is the use of LIBNAME and FILEREF statements, e.g.:


PROC PRINTTO LOG = "&LOGDIR.\data processing.log" NEW;
RUN;
PROC IMPORT 
	DATAFILE= "&DATADIR.\blah.xls"
	OUT = TEMP
	DBMS = EXCEL REPLACE;
    SHEET = "Sheet1"; 
    GETNAMES = YES;
RUN;


Properly setting up SAS Shortcuts in the WinXP environment, or (say)
the UNIX equivalent of batch files in the UNIX environment will cause
SAS to autoexecute (say) an INIT.SAS file that automatically assigns
the LOGDIR and DATADIR macro variables used above.  This is extremely
convenient because it makes the SAS code more portable from project to
project.


MY QUESTION:
Is it possible to use this kind of relative file and directory referencing
from within R?

Kind regards,


      Greg