It looks like they are "Single adjustment positioning"
http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.htm...
So in .fea file:

lookup vpalProportionalAlternateVertic {
lookupflag 0;
pos \A <100 100 200 -300>;
} vpalProportionalAlternateVertic;

lookup paltProportionalAlternateMetric {
lookupflag 0;
pos \B <-200 -300 0 0>;
} paltProportionalAlternateMetric;

BTW palt is Proportional Alternate Widths not Halfwidths

This should help. Below are excerpts from the 'palt' and 'vpal' GPOS features in our KozGoPr6N-Medium OpenType font:

feature palt {
position \634 <-20 0 -500 0>;
position \638 <-250 0 -500 0>;
position \674 <-480 0 -500 0>;
} palt;

feature vpal {
position \638 <0 250 0 -500>;
position \7887 <0 20 0 -500>;
position \7899 <0 480 0 -500>;
} vpal;

The font is CID-keyed, so the glyph references are CIDs. CIDs 634, 638, and 674 correspond to U+3001, U+30FB, and U+FF08, respectively. These are good examples, because they represent a left-justified (U+3001), right-justified (U+FF08), and centered (U+30FB) glyph. The vertical form of CID+634 is CID+7887, and that of CID+674 is CID+7899. CID+638 is used for both horizontal and vertical writing, because it is centered in the em-box.

Although the new advance widths are 500 units, meaning half-width, there is no requirement that this be the case. We simply decided to implement them that way for this particular font.