The lowest note of a Seydel Deluxe Chromatic Irish [slides flat] in G is an F# on the 1 blow with slide in. I would think it might be far the simplest and cheapest way of getting what you seem to be going for. I could be totally wrong, of course, thrown off by a mention of the famous Eddie Clarke and his flipslide playing. Anyway, the Seydel is a stock item, not a custom, and is an excellent harmonica, well made, tuned, and gapped. I bought one from DannyG and I love it.
edited to add stuff.
Thanks, but my goal is not to play in G and then hit the slide for flats, it's to play in F#m, starting with blow rather than draw. If I occasionally play in G, it will probably be all diatonic so it's no hardship to hold the slide in. But I want to play on standard chromatics.
I received the Hering F# (Velvet Voice) today, it sounds great but one flaw; apparently the F# reedplate is taken from the bottom of an F/F# chrome. Therefore, the last draw note is not lower than the blow note, it's higher. So on the highest octave, there's no F natural to resolve the final F#, it's a G instead. So I have two G's up there, and no F natural.
Otherwise it's nice. Live and learn.