Many thanks for this, Wolfgang. That is very helpful. Right now, then, I have:
\definefontfamily[mainface][rm][Brill]
[protrusion=quality,
expansion=quality,
mode=node,
force=yes,
]
\definefallbackfamily [mainface] [rm] [Brill]
[tf=style:italic, range={0x03B8}, offset={0x03D1}]%Greek theta
\definefallbackfamily[mainface][mm][Brill]
[range={0x30-0x39}] %oldstyle numerals, normal
\definefallbackfamily [mainface] [mm] [Brill] [preset=math:lowercasenormal]
\definefallbackfamily [mainface] [mm] [BrillItalic] [preset=math:lowercaseitalic]
\definefallbackfamily [mainface] [mm] [Brill] [preset=math:uppercasenormal]
\definefallbackfamily [mainface] [mm] [BrillItalic] [preset=math:uppercaseitalic]
\definefallbackfamily [mainface] [mm] [Brill] [preset=math:lowercasegreeknormal]
\definefallbackfamily [mainface] [mm] [BrillItalic] [preset=math:lowercasegreekitalic]
\definefallbackfamily [mainface] [mm] [Brill][preset=math:uppercasegreeknormal]
\definefallbackfamily [mainface] [mm] [BrillItalic] [preset=math:uppercasegreekitalic]
\definefontfamily[mainface][mm][TeX Gyre Termes Math][scale=0.94]
\setupmathematics[ucgreek=normal, lcgreek=italic]
\setupbodyfont[mainface,10pt]
\starttext
normal: 1234567890\quad math: \m{1234567890}
normal: ABCDEFabcdef\quad italic: \emph{ABCDEFabcdef}\quad math: \m{\red ABCDEFabcdef}
normal: ΑΒΓΔΕΘαβγδεθ\quad slanted: \emph{ΑΒΓΔΕΘαβγδεθ}\quad math: \m{\red ΑΒΓΔΕΘαβγδεθ}
Greek theta: \emph{θ}
\stoptext
As you can see, all I need is to get the Brill lowercase Greek in mathmode and the proper slanted Greek theta. I have tried this with Pagella and Cambria instead of the Brill font and the problem persists. So I must be doing something wrong. But what?
Alan