The Unicode Range portion of the OS/2 table in TrueType Open fonts does not have bits to indicate support for some blocks in the BMP with displayable code points. The following lists those blocks with the number of Unicode 4.0 defined code points in parentheses:
0500..052F; Cyrillic Supplementary (16)
1720..173F; Hanunoo (23)
1740..175F; Buhid (20)
1760..177F; Tagbanwa (18)
1900..194F; Limbu (66)
1950..197F; Tai Le (35)
19E0..19FF; Khmer Symbols (32)
1D00..1D7F; Phonetic Extensions (108)
27C0..27EF; Miscellaneous Mathematical Symbols-A (28)
27F0..27FF; Supplemental Arrows-A (16)
2900..297F; Supplemental Arrows-B (128)
2980..29FF; Miscellaneous Mathematical Symbols-B (128)
2A00..2AFF; Supplemental Mathematical Operators (256)
2B00..2BFF; Miscellaneous Symbols and Arrows (14)
2E80..2EFF; CJK Radicals Supplement (115)
2F00..2FDF; Kangxi Radicals (214)
2FF0..2FFF; Ideographic Description Characters (12)
3190..319F; Kanbun (16)
31A0..31BF; Bopomofo Extended (24)
31F0..31FF; Katakana Phonetic Extensions (16)
3400..4DB5; CJK Unified Ideographs Extension A (6582)
4DC0..4DFF; Yijing Hexagram Symbols (64)
There is one character below from each of those blocks. Fontswitching will not show any of them except those which are in your Normal font.
Ԅ U+0504 : CYRILLIC CAPITAL LETTER KOMI ZJE
᜶ U+1736 : PHILIPPINE DOUBLE PUNCTUATION
ᝐ U+1750 : BUHID LETTER SA
ᝠ U+1760 : TAGBANWA LETTER A
ᤋ U+190B : LIMBU LETTER TA
ᥕ U+1955 : TAI LE LETTER YA
᧢ U+19E2 : KHMER SYMBOL PII KOET
ᴥ U+1D25 : LATIN LETTER AIN
⟐ U+27D0 : WHITE DIAMOND WITH CENTRED DOT
⟪ U+27EA : MATHEMATICAL LEFT DOUBLE ANGLE BRACKET
⤊ U+290A : UPWARDS TRIPLE ARROW
⦑ U+2991 : LEFT ANGLE BRACKET WITH DOT
⨹ U+2A39 : PLUS SIGN IN TRIANGLE
⬀ U+2B00 : NORTH EAST WHITE ARROW
⺀ U+2E80 : CJK RADICAL REPEAT
⽣ U+2F63 : KANGXI RADICAL LIFE
⿱ U+2FF1 : IDEOGRAPHIC DESCRIPTION CHARACTER ABOVE TO BELOW
㆔ U+3194 : IDEOGRAPHIC ANNOTATION THREE MARK
ㆠ U+31A0 : BOPOMOFO LETTER BU
ㇰ U+31F0 : KATAKANA LETTER SMALL KU
㐀 U+3400 : CJK Ideograph 3400
䷴ U+4DF4 : HEXAGRAM FOR DEVELOPMENT
JWS 2003-08-10