feature ss01 {
sub space A' R' by A_R;
} ss01;
should work.
Edited:
But then it does not work in the beginning of a paragraph.
Maybe with the help of a ignore rule:
feature ss01 {
ignore sub [List of all characters except space, hyphen and a few others] A' R';
sub A' R' by A_R;
} ss01;
I don’t have checked, whether that works.
Edited 2:
Well, now I have checked it. In FontLab Studio it doesn’t seem to work, but according to
http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html#5.f.ii it should work. I am quoting the passage:
Example 2. Matching a beginning-of-word boundary:
ignore substitute @LETTER f' i';
substitute f' i' by f_i.begin;
Maybe you have to compile that with the actual version of the Adobe Font Development Kit for OpenType.
