If you start with a note (or rest), add a time value, then add a multiplication symbol with the fraction of rhythmic value, you will arrive at your desired note.

a1*6/8

So, for instance, a1 means "the note A at the full value of a whole note." But a1*6/8 means "the note A, written as a whole note, but taking up only sixth-eighths of the value of a whole note." This fraction may incidentally be whatever you want.

I've more often used a whole note to indicate a note lasting the entire measure when it's longer than usual (e.g., a1*11/8 to make a whole note fill a measure of 11/8 time; or s1*24 to mean "make an invisible spacer rest that lasts the length of twenty-four whole notes"), but marking a whole note to mean less than the normal value seems out of the ordinary. For 6/8 time, a dotted half note is normal to fill in the measure.

One last note about rests. You do see a "whole" rest fill a measure of any time signature. In Lilypond, that is not represented by r1 --- that will still get you the whole rest taking up its normal value --- but rather by R1 which denotes a "MultiMeasureRest," which will always appear as a whole note, but also be directly centered in the measure.

For 6/8 time, rather than making a whole measure rest r2., I would recommend R1*6/8. You will end up seeing something that looks more like what you normally would encounter.

If you start with a note (or rest), add a time value, then add a multiplication symbol with the fraction of rhythmic value, you will arrive at your desired note.

a1*6/8

So, for instance, a1 means "the note A at the full value of a whole note." But a1*6/8 means "the note A, written as a whole note, but taking up only sixth-eighths of the value of a whole note." This fraction may incidentally be whatever you want.

I've more often used a whole note to indicate a note lasting the entire measure when it's longer than usual (e.g., a1*11/8 to make a whole note fill a measure of 11/8 time; or s1*24 to mean "make an invisible spacer rest that lasts the length of twenty-four whole notes"), but marking a whole note to mean less than the normal value seems out of the ordinary. For 6/8 time, a dotted half note is normal to fill in the measure.

One last note about rests. You do see a "whole" rest fill a measure of any time signature. In Lilypond, that is not represented by r1 --- that will still get you the whole rest taking up its normal value --- but rather by R1 which denotes a "MultiMeasureRest," which will always appear as a whole note, but also be directly centered in the measure.

As found in the documentation on full-measure rests, the number after an uppercase R will denote such a rest. Therefore, since 2. fills the entirety of a 6/8 time signature (as well as 3/4), use R2.. You will end up seeing something that looks more like what you normally would encounter.