Where to place compiled binaries in file system according to File Hierarchy Standart 3.0 ?

${PREFIX}/usr/lib/mono/${EBUILD_FRAMEWORK}

/usr/lib = ${libdir}
for example
/usr/lib/mono/4.5/mydll.dll

egacinstall seems to create symlink in /usr/lib/mono/${PN}/mydll.dll -> /usr/lib/mono/gac/..
.pc files use that symlink

alternative:
/usr/lib/mono/${PN}/slot-${SLOT}/mydll.dll

/use/share/${PN}/slot-${SLOT}/myexe.exe

Newer approach

if [ "${SLOT}" != "0" ]; then
	APPENDIX="-${SLOT}"
fi

# @FUNCTION: framework_assembly_dir
# @DESCRIPTION: returns path for .dll assemblies installation
function framework_assembly_dir() {
	echo "/usr/$(get_libdir)/mono/${FRAMEWORK}/${PN}${APPENDIX}"
}

# @FUNCTION: library_assembly_dir
# @DESCRIPTION:  returns default directory for installing libraries
function library_assembly_dir() {
	echo "/usr/$(get_libdir)/${CATEGORY}/${PN}${APPENDIX}"
}

# @FUNCTION: executable_assembly_dir
# @DESCRIPTION:  returns default directory for installing executables
function executable_assembly_dir() {
	echo "/usr/share/${PN}${APPENDIX}"
}